Question 60. How are you righteous before God?
Answer: Only by true faith in Jesus Christ. That is: although my conscience accuse me, that I have grievously sinned against all the commandments of God, and have never kept any of them, and that I am still prone always to all evil, yet God without any merit of mine, of mere grace, grants and imputes to me the perfect satisfaction, righteousness, and holiness of Christ, as if I had never committed nor had any sin, and had myself accomplished all the obedience which Christ has fulfilled for me, if only I accept such benefit with a believing heart.
Question 62. But why cannot our good works be the whole or part of our righteousness before God?
Answer: Because the righteousness which can stand before the judgment-seat of God, must be perfect throughout and wholly conformable to the divine law; whereas even our best works in this life are all imperfect and defiled with sin.
From The Heidelberg Catechism, Lord’s Day 23 & 24