"For if you forgive other people when they sin against you, your heavenly Father will also forgive you. But if you do not forgive others their sins, your Father will not forgive your sins." Matthew 6:14-15
According to Jesus and the Letter to the Hebrews, the New Testament (Covenant) begins with the blood of Jesus not his birth (Matthew 26:26-29; Luke 22:19-20; Hebrews 9:16-28).
Matthew 6:14-15 are the earthly, pre-cross words of Jesus under the old covenant of law where forgiveness is conditional and requires perfection (70x7-Matthew 18:21-22). Under the new covenant of grace, we forgive others as God has forgiven us in Jesus (Ephesians 4:31-32; Colossians 3:13). These are the heavenly, post-cross words of Jesus to the Apostle Paul to us (Acts 20:24; Galatians 1:11-12; Ephesians 3:2).
Under the old covenant of law, forgiveness is conditional and requires perfection. Under the new covenant of grace, forgiveness is complete because of the perfect sacrifice of Jesus for all of our sins (Hebrews 7:27, 9:11-14, 28, 10:10-18) . As one who has come to faith in Jesus, you are a new creation who lives under the new covenant of grace, not the old covenant of law. Under the new covenant of grace, God no longer counts your sins against you (2 Corinthians 3:1-21). Under grace, we do not forgive to be forgiven. Instead, we forgive because we have been forgiven. If God's forgiveness was still based upon our ability to forgive others perfectly (70x7), then Jesus' death was unnecessary, and none of us could ever be forgiven because none of us are perfect and we all fall short of God's perfection (Romans 3:21).
As the Son of Man, Jesus represented us and fulfilled the law perfectly by living and forgiving perfectly. From the cross he said, "Father forgive them..." He lived perfectly. He forgave perfectly. Through his death, he died for our inability to forgive others. Through his death, we have been forgiven. We now forgive others the way our Lord has forgiven us.