For testimony is given in the Old Testament concerning the Trinity both in figures and in words. Among the figures, however, that one was especially authoritative which was shown to the father of our faith, Abraham: of whom it is said in Genesis eighteen, that when three men appeared to him, he indeed saw three, but adored one, whence he spoke to the three as to one, in this understanding in himself and intimating to others the mystery of the Trinity.
In words, however, that appears especially efficacious for testifying which David, the most excellent of the Prophets, said in the Psalm: By the Word of the Lord the heavens were established, and by the Spirit of His mouth all their power, where the trinity of persons is simultaneously intimated, such that two persons are named explicitly under the name of Spirit and Word, and one implicitly under the name of Lord. Moreover in many places, indeed in nearly all places of the Old Testament, although implicitly, nevertheless through figures and words testimony is rendered to the eternal Trinity.
In this same manner also in the New Testament testimony is given, but explicitly, both in Sacraments and in express teachings. For the first of the Sacraments, which is baptism, according to what is written in the last chapter of Matthew, must be performed in the express invocation of the divine Trinity. For there it is said: Teach all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit: on account of which in that foundational Sacrament the character of the Trinity is imprinted. The words also testify to this same thing, among which, although many there be, that one statement is express which the friend of the Bridegroom wrote in First John five: There are three who give testimony in heaven, the Father, the Word, and the Holy Spirit, and these three are one. Now this testimony is so efficacious and express that it not only makes the aforesaid truth credible, that is, fitting to be believed, but also makes it obligatory, inasmuch as it binds and constrains us to believing it.
But because not all obey the Gospel , and that truth is above reason; therefore divine wisdom has provided an eternal testimony, which indeed is the book of life. Now this book of life, of itself and in itself, explicitly and expressly gives irrefragable testimony to the eternal Trinity to those who with unveiled face see God in the homeland, but on the way it gives testimony according to the influence of light, of which the soul is capable in the state of the way — for as it is said in John 1: The life was the light of men, because this book of life is the true light, which enlightens every man coming into this world. Now it illuminates in a twofold manner, namely through the implanted light and through the infused light, from the concurrence of which lights the habit of faith is elicited as an argument for believing that God is triune, and consequently every truth that pertains to the worship of the Christian religion.
And this is evident as follows. For through the light naturally implanted in man by God and signed as the light of the divine countenance, each one's own reason dictates that concerning the first principle one must think most highly and most piously: most highly, because from none; most piously, because all other things from him. And in this Christians, Jews, and Saracens, and even heretics agree: But that God can and wills to produce one equal and consubstantial to himself, so that he may have an eternal beloved and co-beloved; and that indeed to think this concerning God himself is to think most highly and most piously — because if it is thought that he cannot, one does not think most highly of God; if it is thought that he can but does not will it, one does not think most piously of God — that, I say, it is so and is so thought, this the implanted light does not dictate of itself, but the infused light, from which together with the implanted light it is gathered that one must think of God that he begets and spirates one coequal and consubstantial to himself, so that one may think of God most highly and most piously: and to think this of God is to honor, venerate, and worship God most greatly. And from this it is apparent how faith in the Trinity is both the foundation and root of divine worship and of the entire Christian religion.
From this root, however, by which we believe that one must think of God most highly and most piously, and through this that he himself is triune and one, it is consequently elicited that one must believe God to be the creator of all things, the punisher of the wicked, the rewarder of the good, and the reliever of the wretched. For because he is most high, it therefore befits his dignity to do great and just things. But because he is most pious, it befits his dignity to regard the lowly and raise up the fallen. And hence it is that we believe him incarnate and to have suffered out of supreme piety, and that he will at last judge the world in supreme loftiness and equity.
From what has been said, therefore, it is gathered what the root of the Christian faith is, what its testimonies are, and what also are the things credible from it, and that the principle and first of all credible things which pertain to divine worship is to believe that God is triune and one.
And because the highest and greatest worshipper of all was Jesus Christ, therefore He first commanded that this truth be preached clearly and openly throughout the whole world, thereby binding all to believe it. Whence, since before His coming they were not bound to believe in the Trinity except implicitly, now after the promulgation of the New Testament all are bound to believe it explicitly, so that it is now not only credible because worthy of belief, or fitting from the testimony of creation, but also obligatory through the promulgation of evangelical truth, whose promulgation had its beginning from the Savior, as is said in Hebrews 2, where the Apostle, speaking of the Christian faith, says thus: Having received its beginning to be declared through the Lord, it was confirmed unto us by those who heard, with signs and wonders and various powers and distributions of the Holy Spirit according to His will.
If therefore it is asked: what moves one to believe this, whether namely Scripture, or miracles, or grace, or the eternal truth itself? it must be said that what principally moves to this is illumination itself, which is begun in the implanted light and consummated in the infused light, which indeed causes us to think of God not only loftily, but also piously: and this because illumination proceeds from the eternal light itself, in whose service it captivates our intellect, and by captivating, while it subjects the intellect to God, it worships and venerates Him and renders the intellect apt to believe whatever things pertain to divine honor and worship, even if they are above our reason. This is evident through experience, if anyone should have recourse to the secrets of his own mind.
The motives, however, as supporting and in a certain way inducing, are very many: because the authentic testimonies of Scripture move us, the examples and martyrdoms of the Saints move us, the arguments of the Doctors and the judgment of the universal Church itself move us, and the irrefragable miracles themselves move us. Whence Richard, in the first book On the Trinity, chapter two: "We hold nothing more firmly than what we apprehend by faith: for these things were revealed from heaven to the Fathers and divinely confirmed by signs and wonders so many and so great, so wondrous, that to doubt in these matters even slightly would seem to be grave madness. Innumerable miracles, therefore, and other things which can only be done by divine power, produce faith in these matters and do not permit doubt. We therefore use signs as arguments and wonders as proofs in their attestation and even confirmation. Would that the Jews would attend, would that the pagans would attend, with how great a security of conscience we shall be able to approach the divine judgment on this account! Can we not say to God with all confidence: Lord, if this is an error, we have been deceived by You: for these things have been confirmed in us by such great signs and wonders, and of such a kind, that they can only be done through You. Certainly they have been handed down to us by men of the highest sanctity and proven with the highest and most authentic attestation, with You Yourself 'cooperating and confirming the word with signs following.'"
And through this the response to those four objections which were last raised is clear: what is it that moves the faithful to believe that God is triune and one?
1. To the first objection, therefore: everything whose contrary all reason dictates, etc.; it must be said that one can speak of reason in three ways: either of reason defective, or sound, or elevated. When therefore it is said that everything whose contrary all reason dictates is incredible: if this is understood particularly of defective reason, it is false, because there can be something repugnant to defective reason which nevertheless accords with sound or elevated reason. But if it is understood universally of reason according to its threefold state, then the first proposition is true and must be conceded; but the minor premise is false and must be rejected : because, although defective reason dictates the contrary of that which is the truth, namely that God is triune and one, sound reason nevertheless dictates something which is consonant with this; while elevated reason dictates this very thing, as is clear from what has been said.
2. To that which is objected, that everything whose contrary all created nature exhibits is incredible: it must be said that created nature can exhibit something in two ways: either in itself, or in its cause. For created nature exhibits that it is temporal and mutable and composite, but that its cause is eternal and immutable and most simple. In this same way, although in a creature the unity of form cannot stand together with a trinity of supposits, since supposits in a creature are distinguished by matter and by form appropriated to matter itself; in the Creator, in whom there is absolutely no matter and there is true fecundity, while one form and substance remains, there is distinction in supposits by reason of origin. Therefore, since the creature does not universally exhibit the contrary of this, namely that God is trine, but rather rightly and conformably exhibits the very same: it is therefore more credible than incredible.
3. To that which is objected, that everything which is believed is believed either from reason or beyond reason: it must be said that to believe something from reason is twofold: either from mere reason, or from aided reason. Therefore, although that God is trine is not credible from mere reason, it is nevertheless credible from reason aided by grace and light infused from above. And what is thus credible is not irrationally believed, because grace and light infused from above directs reason rather than perverts it.
4. To that which is objected, that everything which is believed is believed either on account of itself or on account of another: it must be said that that God is trine is believed on account of itself, because it is the highest and first truth pertaining to divine worship.
And to that which is objected, that every such thing credible through itself is evident to the believer: it must be said that this is true of a believer who does not have an obstacle, but rather has a disposition enabling him to assent to uncreated truth itself; and this is the case in the matter at hand, because a true Christian believes this truth proposed to him without any additional persuasion.
5. 6. To that which is objected, that virtue is consonant with nature: it must be said that this is understood of political virtue; or if it is understood of gratuitous virtue, it is not true that it is consonant with nature insofar as it is fallen, but insofar as it is reformed and elevated, which comes about through divine grace infused from above.
And through this the response is clear to that which is consequently objected concerning grace, that grace is the perfection of nature. For grace is a perfection of nature not only enabling, but also reforming and elevating. By reforming moreover and elevating, it does not destroy nature itself, nor anything which belongs to nature itself, but rather the defect surrounding nature.
7. To the objection that the Author of nature does nothing against nature, it must be said that nature is said in two ways: either the natural course, or the natural obedience of the creature. In the second way He does nothing against it, but in the first way He frequently does, as in works which are above nature or contrary to it, such as miraculous works. In this way also one must understand concerning natural intelligence, that there are two ways of speaking about it: either insofar as it is naturally disposed to reason according to its own light, or insofar as it is naturally disposed to subject itself under the eternal light. In this latter way He dictates nothing against intelligence; but in the first way He dictates something which, although it may seem to be contrary, after the infused light becomes consonant: because what previously seemed to reason impossible or difficult to believe, afterwards becomes most easy to believe, so much so that the true believer says to the eternal Teacher Himself: Your testimonies have been made exceedingly credible.
8. To the objection that the principle of scientific knowledge is the easiest to know, etc., it must be said that knowledge according to faith is knowledge according to piety, as is said in Titus 1. And although to the intellect proceeding according to the aridity of speculation it may seem most difficult to believe that God is triune and one, nevertheless to the intellect full of piety it is most easy to hold this concerning the supreme Father: that He has an Only-begotten, whom He loves as He loves Himself, and whom He also delivered for the salvation of men.
9. To the objection that no one is bound to that which he cannot do, it must be said that although man of himself cannot do so sufficiently, he can nevertheless do so with divine assistance, which is at hand for him if he does what is in his power; and because he can do that by which, once done, he can believe, therefore he is judged as capable, and to this he ought to be bound, especially when neither interior nor exterior admonition is lacking — which is indeed the case in the time of revealed grace.
10. To the objection that we are bound either according to the law of nature, or of Scripture, or of grace, it must be said that properly according to the law of grace, in which we are obligated and bound to believe this distinctly. Nevertheless, also in the law of nature and in the law of Scripture there was an obligation to believe, at least implicitly. For at every time whoever would pertinaciously deny that God is triune would be regarded as alien from the truth and from divine worship; but now especially in the time of grace, when this truth has been revealed and preached and approved in manifold ways and expressly. — And if you object that this ought not to be so, because the law of grace should be easy, it must be said that just as loving one's enemy is not easy for anyone whatsoever, but for one having charity, so believing God to be triune, although it may seem difficult to the unbeliever, is nevertheless easy for one having faith.
11. To the objection that if we are bound to believe this, it is either because it is true, or because it is a truth falling under a precept, it must be said that it is because it is a truth falling under a precept.
And if you ask: under which precept? I say that under the first, in which indeed it is said: Hear, O Israel, the Lord your God is one God. You shall adore the Lord your God and Him alone shall you serve. In this precept indeed we are obligated expressly to believe that God is one, but implicitly to believe that God is triune. And this is evident as follows: because by this precept we are bound to worship God with the worship of latria, which is there called service. Now to worship in spiritual matters is according to a transumption from material things: in material things, however, to cultivate a field is to bring forth the shoots proceeding originally from the earth itself. In this same manner, to worship God is to profess by heart and mouth and deed that He is the principle, such that He is not perfectly worshipped unless we confess that He is the principiating principle not only effective in creation, but also restorative in redemption and perfective in glorification: nor yet perfectly, if He is believed to be a principle from time, unless He is believed to be a principiating principle from eternity: nor yet, unless He is believed to be a principle principiating one who eternally principiates: which we believe when we believe that the Father eternally generates the Son, who together with Him spirates the Holy Spirit, so that eternally in God there is a principle from a principle, and a principle not from a principle.
For the perfect worship of God, therefore, it is necessary to hold and believe that He is a principle eternally principiating one who principiates, so that thus there may be a manifestation of the eternal emanations: it is also necessary to believe that He is a creative and re-creative and retributive principle, so that thus there may be a manifestation of the temporal emanations: and whoever is deficient in any of these is not a perfect worshipper of God. And since in the time of grace it is revealed not in just any manner, but perfectly: therefore all these things must be believed explicitly, if we wish to be counted among the number of divine worshippers: and to this we are bound by the very precept of the divine law: which indeed before the coming of Christ bound implicitly, but now explicitly, because it is the time of worshipping God perfectly.