"That every one may give account..." As the passage relates to the recompensing of deeds, we must notice briefly, that, as evil deeds are punished by God, so also good deeds are rewarded, but for a different reason; for evil deeds are requited with the punishment that they deserve, but God in rewarding good deeds does not look to merit or worthiness. For no work is so full and complete in all its parts as to be deservedly well-pleasing to him, and farther, there is no one whose works are in themselves well-pleasing to God, unless he render satisfaction to the whole law. Now no one is found to be thus perfect. Hence the only resource is in his accepting us through unmerited goodness, and justifying us, by not imputing to us our sins. After he has received us into favor, he receives our works also by a gracious acceptance. It is on this that the reward hinges. There is, therefore, no inconsistency in saying, that he rewards good works, provided we understand that mankind, nevertheless, obtain eternal life gratuitously. [emphasis added]Calvin's quote amplifies what is taught in the chapter on good works in Westminster Confession of Faith - Of Good Works, 16:5 & 6.
WCF 16:5. We cannot by our best works merit pardon of sin, or eternal life at the hand of God, by reason of the great disproportion that is between them and the glory to come; and the infinite distance that is between us and God, whom, by them, we can neither profit, nor satisfy for the debt of our former sins, but when we have done all we can, we have done but our duty, and are unprofitable servants: and because, as they are good, they proceed from his Spirit; and as they are wrought by us, they are defiled, and mixed with so much weakness and imperfection, that they cannot endure the severity of God's judgment.
WCF: 16:6. Notwithstanding, the persons of believers being accepted through Christ, their good works also are accepted in him; not as though they were in this life wholly unblamable and unreprovable in God's sight; but that he, looking upon them in his Son, is pleased to accept and reward that which is sincere, although accompanied with many weaknesses and imperfections.