In Matt 6:14-15, we read the following:
For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
A parallel text would be Mark 11:25:
And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses.
These are important texts, as they show that the Christian’s forgiveness of sins is contingent upon them forgiving others, showing that (1) one does not receive a “blanket forgiveness” of sins at justification and (2) shows that one could lose their salvation. As one scholar wrote on the Matthean text stated:
With this logion Matthew repeats the forgiveness petition of the Lord’s Prayer and puts it in parenetic form. Both the conditional wording and the “negative” v. 15, missing from Mark 11:25, make clear that human forgiving is a condition for divine forgiving. Thus with this statement the evangelist emphasizes precisely the part of the Lord’s Prayer where human activity was most directly involved. In contrast to the logion leading into the Lord’s Prayer (vv. 7–8), which emphasizes God’s nearness, this logion that brings the Lord’s Prayer to a close is designed to secure the relationship between prayer and action. Matthew makes clear that prayer is also part of Christian practice, and practice will again be the subject in 6:19–7:27. The forgiveness commandment corresponds in substance to the heart of his ethics, the love commandment. (Ulrich Luz, Matthew 1-7: A Commentary on Matthew 1-7 [Rev. ed.; Minneapolis: Fortress Press, 2007], 327; emphasis added)
