The Heidelberg Catechism: Lord's Day 51 

Question 126: What does the fifth request mean?

 Answer: Forgive us our debts, as we also have forgiven our debtors means, 
Because of Christ’s blood, do not hold against us, poor sinners that we are, any of the sins we do or the evil that constantly clings to us. [1]

 Forgive us just as we are fully determined, as evidence of your grace in us, to forgive our neighbors. [2]

[1] Ps. 51:1-7; 143:2; Rom. 8:1; 1 John 2:1-2; [2] Matt. 6:14-15; 18:21-35

The Heidelberg Catechism: Lord's Day 21

54. Question: What do you believe concerning the holy catholic Christian
church?

Answer: I believe that the Son of God,[1] out of the whole human race,[2] from the beginning of the world to its end,[3] gathers, defends, and preserves for Himself, [4] by His Spirit and Word,[5] in the unity of the true faith,[6] a church chosen to everlasting life.[7] And I believe that I am[8] and forever shall remain a living member of it.[9]

[1] John 10:11; Acts 20:28; Eph. 4:11-13; Col. 1:18. [2] Gen. 26:4; Rev. 5:9. [3] Is. 59:21; I Cor. 11:26. [4] Ps. 129:1-5; Matt. 16:18; John 10:28-30. [5] Rom. 1:16; 10:14-17; Eph. 5:26. [6] Acts 2:42-47; Eph. 4:1-6. [7] Rom. 8:29; Eph. 1:3-14. [8] I John 3:14, 19-21. [9] Ps. 23:6; John 10:27, 28; I Cor. 1:4-9; I Pet. 1:3-5.

55. Question: What do you understand by the communion of saints?

Answer: First, that believers, all and everyone, as members of Christ have communion with Him and share in all His treasures and gifts.[1] Second, that everyone is duty-bound to use his gifts readily and cheerfully for the benefit and well-being of the other members.[2]

[1] Rom. 8:32; I Cor. 6:17; 12:4-7, 12, 13; I John 1:3. [2] Rom. 12:4-8; I Cor. 12:20-27; 13:1-7; Phil. 2:4-8.

56. Question: What do you believe concerning the forgiveness of sins?

Answer: I believe that God, because of Christ's satisfaction, will no more remember my sins,[1] nor my sinful nature, against which I have to struggle all my life,[2] but He will graciously grant me the righteousness of
Christ, that I may never come into condemnation.[3]

[1] Ps. 103:3, 4, 10, 12; Mic. 7:18, 19; II Cor. 5:18-21; I John 1:7; 2:2. [2] Rom. 7:21-25. [3] John 3:17, 18; 5:24; Rom. 8:1, 2.