The promised reply (Ref Ludwig Ott)

By his good works the justified man really acquires a claim to supernatural reward from God.

Note that this is not the same as what you are right now thinking: reward for actions which precede grace, BUT rewards for actions PRECEDED BY GRACE. In other words, grace – specifically antecedent grace- which is UNMERITED, PRECEDES actions in order that they may be accomplished meritoriously.

The Church teaches that for the justified, eternal life is BOTH a gift or grace promised by God AND a reward for his own good works and merits. As God’s grace is the presupposition and foundation of (supernatural) good works, by which man merits eternal life, so salutary works are, AT THE SAME TIME gifts of God AND meritorious acts of man. I’m referring here to TRUE merit, i.e., of meritum de condigno (a technical term).

Sacred Scripture says that eternal blessedness in heaven is the reward for good works performed on this earth, and rewards and merits are correlative concepts . In Mt 5:12 Jesus promises rich rewards In heaven to those who, for his sake, are scorned and persecuted. In Mt 25:34 ff, the Judge of the World decrees eternal reward for the just on the ground of their good works.

In Christ’s discourses, the reward motive frequently recurs (Mt 19:29; Mt 25:21; Luke 6:38. St. Paul, who stresses grace so much also emphasizes the meritorious nature of good works performed with grace—he teaches that the reward is in proportion to the works: “He will render to every man according to his works (Rom 2:6). “Every man shall receive his own reward according to his own labor (1 Cor 3:8). (See also Col 3:24; Heb 10:35; Heb 11:6.)

When Paul characterizes the eternal reward as “the crown of justice which the Lord will render (2 Tim 4:8), he thereby shows the good works of the just establish a legal claim to reward on God (see also Heb 6:10).