Christ takes the opportunity after providing a model prayer to give us a lesson on forgiveness.  He warns that if we are not of a forgiving spirit, then it is clear we are not forgiven.  God does not forgive a sinner and then leave him to bear in his heart and mind anger and grudges against those who have offended him.  The truth is, that if we are constant in prayer, even for those who have sinned against us, it will go very far towards removing the enmity we might feel towards them.

Our Lord is not here teaching a system of merit by works, for otherwise He would contradict both His own and the teaching of His apostles.  But the genuine meaning here is that those who do not forgive men their trespasses have never received divine forgiveness.  On the good side of the coin, those who do manifest a forgiving spirit, even towards their bitterest enemies, have gone very far towards manifesting the fruits of divine grace working in their hearts.