With the merciful - From the particular statement respecting the divine dealings with himself the psalmist now passes to a general statement (suggested by what God had done for him) in regard to the general principles of the divine administration. That general statement is, that God deals with men according to their character; or, that he will adapt his providential dealings to the conduct of men. They will find him to be such toward them as they have shown themselves to be toward him. The word merciful refers to one who is disposed to show kindness or compassion to those who are guilty, or to those who injure or wrong us.
Thou wilt show thyself merciful - Thou wilt evince toward him the same character which he shows to others. It is in accordance with this that the Saviour teaches us to pray, “And forgive us our debts, as we forgive our debtors,” Matthew 6:12. And in accordance also with this he said, “For if ye forgive men their trespasses, your heavenly Father will also forgive you: but if ye forgive not men their trespasses, neither will your Father forgive your trespasses,” Matthew 6:14-15.
With an upright man - literally, a perfect man. See Job 1:1, where the same word is used in the original, and rendered perfect. The idea is that of a man who is consistent, or whose character is complete in all its parts. See the note at Job 1:1.
Thou wilt show thyself upright - Thou wilt deal with him according to his character. As he is faithful and just, so will he find that he has to do with a God who is faithful and just.