Reply Obj. 2: Anyone may congruously merit for another his first grace, because there is no impediment (at least, on the part of him who merits), such as is found when anyone recedes from justice after the merit of grace.
Reply Obj. 3: Some have said that no one absolutely merits life everlasting except by the act of final grace, but only conditionally, i.e. if he perseveres. But it is unreasonable to say this, for sometimes the act of the last grace is not more, but less meritorious than preceding acts, on account of the prostration of illness. Hence it must be said that every act of charity merits eternal life absolutely; but by subsequent sin, there arises an impediment to the preceding merit, so that it does not obtain its effect; just as natural causes fail of their effects on account of a supervening impediment. ________________________
EIGHTH ARTICLE [I-II, Q. 114, Art. 8]
Whether a Man May Merit the Increase of Grace or Charity?
Objection 1: It would seem that a man cannot merit an increase of grace or charity. For when anyone receives the reward he merited no other reward is due to him; thus it was said of some (Matt. 6:2): "They have received their reward." Hence, if anyone were to merit the increase of charity or grace, it would follow that, when his grace has been increased, he could not expect any further reward, which is unfitting.
Obj. 2: Further, nothing acts beyond its species. But the principle of merit is grace or charity, as was shown above (AA. 2, 4). Therefore no one can merit greater grace or charity than he has.
Obj. 3: Further, what falls under merit a man merits by every act flowing from grace or charity, as by every such act a man merits life everlasting. If, therefore, the increase of grace or charity falls under merit, it would seem that by every act quickened by charity a man would merit an increase of charity. But what a man merits, he infallibly receives from God, unless hindered by subsequent sin; for it is written (2 Tim. 1:12): "I know Whom I have believed, and I am certain that He is able to keep that which I have committed unto Him." Hence it would follow that grace or charity is increased by every meritorious act; and this would seem impossible since at times meritorious acts are not very fervent, and would not suffice for the increase of charity. Therefore the increase of charity does not come under merit.
On the contrary, Augustine says (super Ep. Joan.; cf. Ep. clxxxvi) that "charity merits increase, and being increased merits to be perfected." Hence the increase of grace or charity falls under merit.
I answer that, As stated above (AA. 6, 7), whatever the motion of grace reaches to, falls under condign merit. Now the motion of a mover extends not merely to the last term of the movement, but to the whole progress of the movement. But the term of the movement of grace is eternal life; and progress in this movement is by the increase of charity or grace according to Prov. 4:18: "But the path of the just as a shining light, goeth forward and increaseth even to perfect day," which is the day of glory. And thus the increase of grace falls under condign merit.
Reply Obj. 1: Reward is the term of merit. But there is a double term of movement, viz. the last, and the intermediate, which is both beginning and term; and this term is the reward of increase. Now the reward of human favor is as the last end to those who place their end in it; hence such as these receive no other reward.