The Heidelberg Catechism, This Lord's Day week 21

Q54: What do you believe concerning the "Holy Catholic Church"?

A54: That out of the whole human race,[1] from the beginning to the end of the world,[2] the Son of God,[3] by His Spirit and Word,[4] gathers, defends and preserves for Himself unto everlasting life a chosen communion [5] in the unity of the true faith;[6] and that I am and forever shall remain a living member of this communion.[7]
1. Gen. 26:42. John 10:103. Eph. 1:10-134. Rom. 1:16; 10:14-17; Isa. 59:21; Eph. 5:265. Rom. 8:29-30; Matt. 16:18; Eph. 4:3-66. Acts 2:46; Psa. 71:18; I Cor. 1:8-9; 11:26; John 10:28-307. I John 2:19; 3:21; Gal. 3:28

Q55: What do you understand by the "communion of saints"?

A55: First, that believers, one and all, as members of the Lord Jesus Christ, are partakers with Him in all His treasures and gifts;[1] second, that each one must feel himself bound to use his gifts readily and cheerfully for the advantage and welfare of other members.[2]
1. I John 1:32. I Cor. 12:12-13, 21; 13:5-6; Phil. 2:4-6; Heb. 3:14

Q56: What do you believe concerning the "forgiveness of sins"?

A56: That God, for the sake of Christ's satisfaction,[1] will no more remember my sins, nor the sinful nature with which I have to struggle all my life long;[2] but graciously imputes to me the righteousness of Christ, that I may nevermore come into condemnation.[3]
1. I John 2:22. II Cor. 5:19, 21; Rom. 7:24-25; 8:1-4; Psa. 103:3, 10, 12; Jer. 31:343. John. 3:18; Eph. 1:7; Rom. 4:7-8; 7:18