stated above (A.
1), human acts have the nature of merit from two causes: first and
chiefly from the Divine ordination, inasmuch as acts are said to
merit that good to which man is divinely ordained. Secondly, on the
part of free-will, inasmuch as man, more than other creatures, has
the power of voluntary acts by acting by himself. And in both these
ways does merit chiefly rest with charity. For we must bear in mind
that everlasting life consists in the enjoyment of God. Now the human
mind's movement to the fruition of the Divine good is the proper act
of charity, whereby all the acts of the other virtues are ordained to
this end, since all the other virtues are commanded by charity. Hence
the merit of life everlasting pertains first to charity, and
secondly, to the other virtues, inasmuch as their acts are commanded
by charity. So, likewise, is it manifest that what we do out of love
we do most willingly. Hence, even inasmuch as merit depends on
voluntariness, merit is chiefly attributed to charity.

Reply Obj. 1: Charity, inasmuch as it has the last end for object,
moves the other virtues to act. For the habit to which the end
pertains always commands the habits to which the means pertain, as
was said above (Q. 9, A. 1).

Reply Obj. 2: A work can be toilsome and difficult in two ways:
first, from the greatness of the work, and thus the greatness of the
work pertains to the increase of merit; and thus charity does not
lessen the toil--rather, it makes us undertake the greatest toils,
"for it does great things, if it exists," as Gregory says (Hom. in
Evang. xxx). Secondly, from the defect of the operator; for what is
not done with a ready will is hard and difficult to all of us, and
this toil lessens merit and is removed by charity.

Reply Obj. 3: The act of faith is not meritorious unless "faith . . .
worketh by charity" (Gal. 5:6). So, too, the acts of patience and
fortitude are not meritorious unless a man does them out of charity,
according to 1 Cor. 13:3: "If I should deliver my body to be burned,
and have not charity, it profiteth me nothing."
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FIFTH ARTICLE [I-II, Q. 114, Art. 5]

Whether a Man May Merit for Himself the First Grace?

Objection 1: It would seem that a man may merit for himself the first
grace, because, as Augustine says (Ep. clxxxvi), "faith merits
justification." Now a man is justified by the first grace. Therefore
a man may merit the first grace.

Obj. 2: Furth