The Sermon on the Mount is a penetrating discussion about relationships—between God and people of faith in God. God, through Christ our Savior, is teaching His own (for the most part) what it means to be in relationship with Him and with others. With this premise in mind, perhaps we need to look at God’s dealings with His children throughout the Scripture.
The underlying ethic in Jesus’ teaching in the Sermon on the Mount is love: love for our heavenly Father and love for people. As we have seen God, in His love for us, allows us to come face to face with our sin, to be confronted by it. This of course is to bring about deep repentance and restoration of the love between the parties involved.
While all the commentators surveyed were quite consistent in saying that an unforgiving Christian does lose fellowship with God, they did not explicitly say what God is attempting to do by not forgiving the person. Therefore, His unwillingness to forgive (and despite what the commentators say, the text explicitly says that He will not forgive) the person is an act motivated by love and a deep-seated commitment to move towards His sinning child and not away from him.
Therefore, when Jesus says that the Father will not forgive, what He means is that God will allow the person to walk in their sin to the necessary extent; until they come face to face with it and see it for what it is. In other words, if the person is unwilling to forgive, let him deal with a God twice as stubborn when it comes to forgiving. God will not give in and the sinning brother will have to deal with an unforgiving Father, from whom he depends for the basic necessities of life (6:25-32).
God’s intention, as the unforgiving brother goes his way in unforgiveness, is to expose him, to bring about legitimate shame and repulsiveness towards the sin. It is a rare blend of justice and mercy. He did this for Jacob, for Israel and as Paul declares, for us also. The result will often be Spirit-inspired, genuine repentance and love.