Strict, condign, and congruent merit defined

Strict merit denotes a contractual relationship between two parties in which one party performs work for the other party and thus is owed a compensation for the work done. The paying party is under legal obligation to pay the working party. Condign or congruent merit denotes a non-contractual relationship between two parties in which neither the party doing the work demands payment from the party for whom the work is done, nor can the party for whom the work is performed be under obligation to pay for the work. Whatever is given to the first party for the work done is only by the sheer gratuity of the second party. In other words, the party for whom work is done may pay the worker out of kindness, but he is not obligated to do so. Specifically, condign merit (Latin: meritum de condigno) is that which is gratuitously rewarded to an individual in keeping with his obedience. The reward only applies to the just. It can be an increase in sanctifying grace or an increase in heavenly glory (cf. Hb 6:10; 1Tm6:18-19; 2Tm 4:8). Congruent merit (Latin: meritum de congruo) is merit given in accord with the act performed but far outweighing the act itself, either to the just or the unjust (cf. Ps 50:19; Lk 18:9-14; 1Tm1:13-14). (Robert A. Sungenis, Not by Faith Alone: The Biblical Evidence for the Catholic Doctrine of Justification [2d ed.; Catholic Apologetics International Publishing Inc., 2009], 600, italics in original)