Heidelberg Catechism Lord's Day 23 (section Our Justification)

59: Q. But what does it help you now that you believe all this?

A. In Christ I am righteous before God and heir to life everlasting.

60: Q. How are you righteous before God?

A. Only by true faith in Jesus Christ. Although my conscience accuses me that I have grievously sinned against all God's commandments, have never kept any of them, and am still inclined to all evil, yet God, without any merit of my own, out of mere grace, imputes to me the perfect satisfaction, righteousness, and holiness of Christ.  He grants these to me as if I had never had nor committed any sin, and as if I myself had accomplished all the obedience which Christ has rendered for me, if only I accept this gift with a believing heart.

61: Q. Why do you say that you are righteous only by faith?

A. Not that I am acceptable to God on account of the worthiness of my faith, for only the satisfaction, righteousness, and holiness of Christ is my righteousness before God.  I can receive this righteousness and make it my own by faith only.