"But if ye forgive not men their trespasses, neither will your Father forgive your trespasses." (Matthew 6:15)
Ouch! I though that the forgiveness of God was everlasting?!
"Fear not; for thou shalt not be ashamed: neither be thou confounded; for thou shalt not be put to shame: for thou shalt forget the shame of thy youth, and shalt not remember the reproach of thy widowhood any more." (Isaiah 54:4)
And then:
"And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more." (Jeremiah 31:34)
Which the writer of Hebrews quoted:
"For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more." (Hebrews 8:12)
The writer was so enthusiastic about this promised, realized through the death of Jesus on the Cross, that he repeated it:
"And their sins and iniquities will I remember no more." (Hebrews 10:17)
So, what is the cause of the apparent conflict from Jesus' stern admonition in the Sermon on the Mount?
Simple: Jesus had not died yet! He had not suffered the wrath of His Father on the Cross for the sins of the world.
Consider the glorious calm of the Christ Risen when he met his disciples in the upper room:
"Jesus and stood in the midst, and saith unto them, Peace be unto you. "(John 20:19)
Then Jesus said:
"Peace be unto you: as my Father hath sent me, even so send I you.
"And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost:
"Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained." (John 20:21-23)
Because Christ dies on the Cross, reconciling us to His Father, He now gave to his disciples, both then, now, and forever, the power to forgive sins! Before His death, we could not forgive, or release others who had harmed u.
No, because of the Finished Work, we have the power to forgive! In effect, we forgive because we have been forgiven!
"Wherefore I say unto thee, Her sins, which are many, are forgiven; for she loved much: but to whom little is forgiven, the same loveth little." (Luke 7:47)
And
"And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened together with him, having forgiven you all trespasses;" (Colossians 2:13)
Paul follows this blessed legacy with the following:
"Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also do ye." (Colossians 3:13)
Paul does not give a stern warning, but a calm entreaty to forgive, for we in turn have been forgiven!
So, in effect, believers have to forgive, not so much because of the consequences for not doing so, but as a consequence for having been forgiven all unrighteousness at the Cross!