Q. 62. But why cannot our good works be the whole, or part of our righteousness before God?
Because, that the righteousness, which can be approved of before the tribunal of God, must be absolutely perfect, and in all respects conformable to the divine law; and also, that our best works in this life are all imperfect and defiled with sin.
 Galatians 3:10; Deuteronomy 27:26;  Isaiah 64:6.
Q. 63. What! do not our good works merit, which yet God will reward in this and in a future life?
This reward is not of merit, but of grace.
 Luke 17:10.
Q. 64. But doth not this doctrine make men careless and profane?
By no means: for it is impossible that those, who are implanted into Christ by a true faith, should not bring forth fruits of thankfulness.
 Matthew 7:17–18; John 15:5.
We have seen that our justification or acceptance before God, for eternal life is entirely by grace through faith. It is neither through good works nor keeping of God’s Law. The Apostle proves this doctrine in his letter to the Romans by showing that all men are sinners: there are none righteous and all have fallen short of the glory of God (Rom 3:10–23). If that is the case, then all attempts at doing good works will fail before the absolute holiness of God, seeing that so long as the heart has any remnant of corruption, our motives will be corrupted and therefore our deeds will be defiled. This Paul contends definitely: “Therefore we conclude that a man is justified by faith without the deeds of the law” (Rom 3:28).
But this doctrine does raise a few legitimate questions. In the first place, does not the Scripture teach that God will reward His saints (Mt 5:12; 16:27; Mk 9:41; 1 Cor 3:8, 14; Rev 22:12; etc.)? If that is the case, then how could our good works be not good enough to be meritorious? Does not God reward us because we deserve reward for our good works? Well, there is no doubt that God will reward His saints, but we must not forget that God is not a debtor to us to reward us when our good works do fall short of His perfection. Any reward that God gives to us, then, must be reckoned not as being meritorious or deserving of God’s reward, but as being of grace. Of grace because of our union with Christ and because the righteousness of Christ has been imputed to us. The WCF has perhaps the most excellent confessional statement on this doctrine ever penned:
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 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.
Heidelberg Catechism >