Forgive those that wrong you, before you pray
"And whenever you stand praying, if you have anything against anyone, forgive him, that your Father in heaven may also forgive you your trespasses. But if you do not forgive, neither will your Father in heaven forgive your trespasses."
"The sense is, that if, while a man is praying, it comes into his mind that such an one has committed a trespass against him, has done him an injury, of which he has just reason to complain. But instead of complaining of it before God, and calling upon him to avenge his cause, he should immediately in his heart, and from his heart, forgive him, even though he is not present to acknowledge his sin, and ask his pardon.
Such an one may expect forgiveness of God, and a manifestation of it to his soul; which is one [of] the things he is constantly praying for, as his daily case makes it necessary.
[Note that it is ]not that it is to be understood that forgiving the person that has offended him, is the cause, or condition, of his receiving remission of sin at the hand of God. For then it would not be through the blood of Christ, and according to the riches of his grace; but [rather] this points at a temper and disposition of mind well pleasing to God, and describes persons who may expect this favour from him"(John Gill)