CLAIM: Jesus said, “If you do not forgive others, then your Father will not forgive your transgressions” (Mt. 6:15). Catholic apologist Tim Staples writes, “In Matthew 6:15, Jesus tells us that ‘if you do not forgive men their trespasses, neither will your Father forgive your trespasses.’ It does not matter how ‘born again’ one may be or how many experiences one has had, if he does not forgive others, he will not be forgiven, according to the text.” Is forgiveness conditional or unconditional?
RESPONSE: Jesus’ sermon was given under the old covenant law. While forgiveness was conditional in the old covenant, it is unconditional in the new. Since Christ fulfilled the law for us (Mt. 5:17; Rom. 10:4), we are no longer obligated to keep the law as believers. Paul writes, “You are not under law but under grace” (Rom. 6:14). Elsewhere he writes, “We have been released from the Law, having died to that by which we were bound, so that we serve in newness of the Spirit and not in oldness of the letter” (Rom. 7:6). Since we have been forgiven by virtue of the Cross of Christ, we forgive based on the forgiveness of Christ.
(Eph. 4:32) Be kind to one another, tender-hearted, forgiving each other, just as God in Christ also has forgiven you.
(Col. 3:13) Bearing with one another, and forgiving each other, whoever has a complaint against anyone; just as the Lord forgave you, so also should you.
For more keys to interpreting the Sermon on the Mount, see comments on Matthew 5:1.
 Staples, Tim. “Are You Saved? If Only” This Rock. Volume 20. Number 3. 2009.