Objection 1. It would seem that lifeless faith does not become living, or living faith lifeless. For, according to 1 Corinthians 13:10, "when that which is perfect is come, that which is in part shall be done away." Now lifeless faith is imperfect in comparison with living faith. Therefore when living faith comes, lifeless faith is done away, so that they are not one identical habit.
Objection 2. Further, a dead thing does not become a living thing. Now lifeless faith is dead, according to James 2:20: "Faith without works is dead." Therefore lifeless faith cannot become living.
Objection 3. Further, God's grace, by its advent, has no less effect in a believer than in an unbeliever. Now by coming to an unbeliever it causes the habit of faith. Therefore when it comes to a believer, who hitherto had the habit of lifeless faith, it causes another habit of faith in him.
Objection 4. Further, as Boethius says (In Categ. Arist. i), "accidents cannot be altered." Now faith is an accident. Therefore the same faith cannot be at one time living, and at another, lifeless.
On the contrary, A gloss on the words, "Faith without works is dead" (James 2:20) adds, "by which it lives once more." Therefore faith which was lifeless and without form hitherto, becomes formed and living.
I answer that, There have been various opinions on this question. For some [William of Auxerre, Sum. Aur. III, iii, 15 have said that living and lifeless faith are distinct habits, but that when living faith comes, lifeless faith is done away, and that, in like manner, when a man sins mortally after having living faith, a new habit of lifeless faith is infused into him by God. But it seems unfitting that grace should deprive man of a gift of God by coming to him, and that a gift of God should be infused into man, on account of a mortal sin.
Consequently others [Alexander of Hales, Sum. Theol. iii, 64 have said that living and lifeless faith are indeed distinct habits, but that, all the same, when living faith comes the habit of lifeless faith is not taken away, and that it remains together with the habit of living faith in the same subject. Yet again it seems unreasonable that the habit of lifeless faith should remain inactive in a person having living faith.
We must therefore hold differently that living and lifeless faith are one and the same habit. The reason is that a habit is differentiated by that which directly pertains to that habit. Now since faith is a perfection of the intellect, that pertains directly to faith, which pertains to the intellect. Again, what pertains to the will, does not pertain directly to faith, so as to be able to differentiate the habit of faith. But the distinction of living from lifeless faith is in respect of something pertaining to the will, i.e. charity, and not in respect of something pertaining to the intellect. Therefore living and lifeless faith are not distinct habits.
Reply to Objection 1. The saying of the Apostle refers to those imperfect things from which imperfection is inseparable, for then, when the perfect comes the imperfect must needs be done away. Thus with the advent of clear vision, faith is done away, because it is essentially "of the things that appear not." When, however, imperfection is not inseparable from the imperfect thing, the same identical thing which was imperfect becomes perfect. Thus childhood is not essential to man and consequently the same identical subject who was a child, becomes a man. Now lifelessness is not essential to faith, but is accidental thereto as stated above. Therefore lifeless faith itself becomes living.
Reply to Objection 2. That which makes an animal live is inseparable from an animal, because it is its substantial form, viz. the soul: consequently a dead thing cannot become a living thing, and a living and a dead thing differ specifically. On the other hand that which gives faith its form, or makes it live, is not essential to faith. Hence there is no comparison.
Reply to Objection 3. Grace causes faith not only when faith begins anew to be in a man, but also as long as faith lasts. For it has been said above (I, 104, 1; I-II, 109, 9) that God is always working man's justification, even as the sun is always lighting up the air. Hence grace is not less effective when it comes to a believer than when it comes to an unbeliever: since it causes faith in both, in the former by confirming and perfecting it, in the latter by creating it anew.
We might also reply that it is accidental, namely on account of the disposition of the subject, that grace does not cause faith in one who has it already: just as, on the other hand, a second mortal sin does not take away grace from one who has already lost it through a previous mortal sin.
Reply to Objection 4. When living faith becomes lifeless, faith is not changed, but its subject, the soul, which at one time has faith without charity, and at another time, with charity.
Source: Summa Theologica (New Advent)