William Burkitt on Matthew 6:14

There being no duty to which our corrupt natures are more backward than this of forgiving injuries, our Saviour repeats that duty over and over, and frequently inculcates it in the holy Gospels; assuring us, that forgiving others is the indispensable condition upon which we are to expect forgiveness from God.

Learn thence, That every time we go to God in prayer, and beg forgiveness of him, as we forgive others; if we do not forgive them heartily and sincerely, fully and freely, readily and willingly, we fly in the face of God, and our prayers are a sort of imprecations against ourselves.

Note farther, That although God promises us forgiveness if we forgive others; yet it is with this limitation, of no other condition of salvation be wanting, for this virtue alone cannot obtain favour with God, unless other duties are performed.