Of Prayer: Which is the fifth petition?

Heidelberg Catechism (extended)

The Catechism
Method of Instruction in the Christian Religion
As the Same is Taught in the Reformed Churches and Schools

(with the Scripture references written out)

Note. This Catechism is fully based on the Scriptures. The references to Scripture are indicated in parentheses with a letter. For example, the letter (a) points to the texts (a) placed after the answer.

Of Prayer

51. Lord's Day 

 Q. 126. Which is the fifth petition? 

A. "And forgive us our debts as we forgive our debtors"; that is, be pleased for the sake of Christ's blood, not to impute to us poor sinners, our transgressions, nor that depravity, which always cleaves to us; (a) even as we feel this evidence of thy grace in us, that it is our firm resolution from the heart to forgive our neighbour. (b) 

 (a) Ps.51:1 To the chief Musician, A Psalm of David, when Nathan the prophet came unto him, after he had gone in to Bathsheba. Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions. 

Ps.51:2 Wash me throughly from mine iniquity, and cleanse me from my sin. 

Ps.51:3 For I acknowledge my transgressions: and my sin is ever before me. 

Ps.51:4 Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest. 

Ps.51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me. 

Ps.51:6 Behold, thou desirest truth in the inward parts: and in the hidden part thou shalt make me to know wisdom. 

Ps.51:7 Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow. Ps.143:2 And enter not into judgment with thy servant: for in thy sight shall no man living be justified. 

1 John 2:1 My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: 

1 John 2:2 And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world. 

Rom.8:1 There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit. 

(b) Matt.6:14 For if ye forgive men their trespasses, your heavenly Father will also forgive you:

 Matt.6:15 But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.

Source: http://www.ccel.org/creeds/heidelberg-cat-ext.txt