In Christian theology, merit (Latin: meritum) is a good work done that is "seen to have a claim to a future reward from a graceful God".
Within Christianity, both Catholics and Lutherans affirm that "By grace alone, in faith in Christ's saving work and not because of any merit on our part, we are accepted by God and receive the Holy Spirit, who renews our hearts while equipping and calling us to good works". The Catholic Church further teaches that "When Catholics affirm the 'meritorious' character of good works, they wish to say that, according to the biblical witness, a reward in heaven is promised to these works. Their intention is to emphasize the responsibility of persons for their actions, not to contest the character of those works as gifts, or far less to deny that justification always remains the unmerited gift of grace".
Roman Catholicism "speaks of merit in three distinct ways": condign merit (which God crowns freely), congruous merit (nonobligatory reward that may result in sanctifying grace), and supererogatory merit (given for doing above what a Christian is required).Reformed doctrine, on the other hand, puts more emphasis on the merit of Christ that humans receive through divine grace.
In Catholic philosophy, merit (as understood to be a property of a good work which entitles the doer to receive a reward), is a salutary act to which God, in whose service the work is done, in consequence of his infallible promise may give a reward (prœmium, merces).
Merit exists only in works that are positively good. The relation between merit and reward furnishes the intrinsic reason why in the matter of service and its remuneration, the guiding norm can be only the virtue of justice, and not disinterested kindness or pure mercy; for it would destroy the very notion of reward to conceive of it as a free gift of bounty (cf. Rom., xi, 6). If, however, salutary acts can in virtue of divine justice give the right to an eternal reward, this is possible only because they themselves have their root in gratuitous grace, and consequently are of their very nature dependent ultimately on grace, as the Council of Trent emphatically declares (Sess. VI, cap. xvi, in Denzinger, 10th ed., Freiburg, 1908, n. 810): "the Lord ... whose bounty towards all men is so great, that He will have the things, which are His own gifts, be their merits."