In the second way miracles are not wrought except by the saints, since it is in proof of their holiness that miracles are wrought during their lifetime or after death, either by themselves or by others. For we read ( Acts 19:11 , 12 ) that "God wrought by the hand of Paul . . . miracles" and "even there were brought from his body to the sick, handkerchiefs . . . and the diseases departed from them." In this way indeed there is nothing to prevent a sinner from working miracles by invoking a saint; but the miracle is ascribed not to him, but to the one in proof of whose holiness such things are done.
Reply to Objection 1: As stated above (Q[83], A[16]) when we were treating of prayer, the prayer of impetration relies not on merit but on God's mercy, which extends even to the wicked, wherefore the prayers even of sinners are sometimes granted by God. Hence Augustine says (Tract. xliv in Joan.) that "the blind man spoke these words before he was anointed," that is, before he was perfectly enlightened; "since God does hear sinners." When it is said that the prayer of one who hears not the law is an abomination, this must be understood so far as the sinner's merit is concerned; yet it is sometimes granted, either for the spiritual welfare of the one who prays---as the publican was heard ( Lk. 18:14 )---or for the good of others and for God's glory.
Reply to Objection 2: Faith without works is said to be dead, as regards the believer, who lives not, by faith, with the life of grace. But nothing hinders a living thing from working through a dead instrument, as a man through a stick. It is thus that God works while employing instrumentally the faith of a sinner.
Reply to Objection 3: Miracles are always true witnesses to the purpose for which they are wrought. Hence wicked men who teach a false doctrine never work true miracles in confirmation of their teaching, although sometimes they may do so in praise of Christ's name which they invoke, and by the power of the sacraments which they administer. If they teach a true doctrine, sometimes they work true miracles as confirming their teaching, but not as an attestation of holiness. Hence Augustine says (QQ. lxxxiii, qu. 79): "Magicians work miracles in one way, good Christians in another, wicked Christians in another. Magicians by private compact with the demons, good Christians by their manifest righteousness, evil Christians by the outward signs of righteousness."
Reply to Objection 4: As Augustine says (QQ. lxxxiii, qu. 79), "the reason why these are not granted to all holy men is lest by a most baneful error the weak be deceived into thinking such deeds to imply greater gifts than the deeds of righteousness whereby eternal life is obtained."