Objection 1. It would seem that lifeless faith is not a gift of God. For it is written (Deuteronomy 32:4) that "the works of God are perfect." Now lifeless faith is something imperfect. Therefore it is not the work of God.
Objection 2. Further, just as an act is said to be deformed through lacking its due form, so too is faith called lifeless [informis] when it lacks the form due to it. Now the deformed act of sin is not from God, as stated above (I-II, 79, 2, ad 2). Therefore neither is lifeless faith from God.
Objection 3. Further, whomsoever God heals, He heals wholly: for it is written (John 7:23): "If a man receive circumcision on the sabbath-day, that the law of Moses may not be broken; are you angry at Me because I have healed the whole man on the sabbath-day?" Now faith heals man from unbelief. Therefore whoever receives from God the gift of faith, is at the same time healed from all his sins. But this is not done except by living faith. Therefore living faith alone is a gift of God: and consequently lifeless faith is not from God.
On the contrary, A gloss on 1 Corinthians 13:2 says that "the faith which lacks charity is a gift of God." Now this is lifeless faith. Therefore lifeless faith is a gift of God.
I answer that, Lifelessness is a privation. Now it must be noted that privation is sometimes essential to the species, whereas sometimes it is not, but supervenes in a thing already possessed of its proper species: thus privation of the due equilibrium of the humors is essential to the species of sickness, while darkness is not essential to a diaphanous body, but supervenes in it. Since, therefore, when we assign the cause of a thing, we intend to assign the cause of that thing as existing in its proper species, it follows that what is not the cause of privation, cannot be assigned as the cause of the thing to which that privation belongs as being essential to its species. For we cannot assign as the cause of a sickness, something which is not the cause of a disturbance in the humors: though we can assign as cause of a diaphanous body, something which is not the cause of the darkness, which is not essential to the diaphanous body.
Now the lifelessness of faith is not essential to the species of faith, since faith is said to be lifeless through lack of an extrinsic form, as stated above (Question 4, Article 4). Consequently the cause of lifeless faith is that which is the cause of faith strictly so called: and this is God, as stated above (Article 1). It follows, therefore, that lifeless faith is a gift of God.
Reply to Objection 1. Lifeless faith, though it is not simply perfect with the perfection of a virtue, is, nevertheless, perfect with a perfection that suffices for the essential notion of faith.
Reply to Objection 2. The deformity of an act is essential to the act's species, considered as a moral act, as stated above (I, 48, 1, ad 2; I-II, 18, 5): for an act is said to be deformed through being deprived of an intrinsic form, viz. the due commensuration of the act's circumstances. Hence we cannot say that God is the cause of a deformed act, for He is not the cause of its deformity, though He is the cause of the act as such.
We may also reply that deformity denotes not only privation of a due form, but also a contrary disposition, wherefore deformity is compared to the act, as falsehood is to faith. Hence, just as the deformed act is not from God, so neither is a false faith; and as lifeless faith is from God, so too, acts that are good generically, though not quickened by charity, as is frequently the case in sinners, are from God.
Reply to Objection 3. He who receives faith from God without charity, is healed from unbelief, not entirely (because the sin of his previous unbelief is not removed) but in part, namely, in the point of ceasing from committing such and such a sin. Thus it happens frequently that a man desists from one act of sin, through God causing him thus to desist, without desisting from another act of sin, through the instigation of his own malice. And in this way sometimes it is granted by God to a man to believe, and yet he is not granted the gift of charity: even so the gift of prophecy, or the like, is given to some without charity.
Source: Summa Theologica (New Advent)