2 It is then for our instruction that He put forth this parable, and by this warning He would save us from perishing. “So,” said He, “shall My heavenly Father do also unto you, if you from your hearts forgive not every one his brother their trespasses.” Lo, Brethren, the thing is plain, useful is the admonition, and a wholesome obedience is by all means due, that what has been bidden may be fulfilled. For every man is at once God's debtor, and has also some brother a debtor to himself.
For who is there who is not God's debtor, but he in whom there can be found no sin? And who is there who has not a brother his debtor, but he against whom no one has sinned? Think you that any one among mankind can be found, who is not himself bounden to his brother by some sin? So then every man is a debtor, yet having himself his own debtors too. The righteous God therefore appoints a rule for you toward your debtor, which He also will observe with His. For two works of mercy are there, which deliver us, which the Lord has Himself briefly laid down in the Gospel: “Forgive, and you shall be forgiven: give, and it shall be given unto you.” “Forgive, and you shall be forgiven,” relates to pardoning.
“Give, and it shall be given unto you,” relates to doing kindnesses. As to what He says of pardoning, you both wish your sin to be pardoned you, and you have another whom you may pardon. Again, as to the doing kindnesses; a beggar asks of you, and you are God's beggar. For we are all when we pray God's beggars; we stand, yea rather we fall prostrate before the door of the Great Householder, we groan in supplication wishing to receive something; and this something is God Himself. What does the beggar ask of you? Bread. And what do you ask of God, but Christ, who says, “I am the living Bread which came down from heaven”? Would you be forgiven? Forgive. “Forgive, and it shall be forgiven you.” Would you receive? “Give, and it shall be given unto you.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)