We can pray Psalm 17 only because of the free gift of righteousness in our Lord Jesus Christ. Anyone who prays verses 3-4 apart from Christ does not know what he is saying:
You examine my heart. / You visit me at night. / You refine me. / You will find nothing. / I have purposed that my mouth will not transgress. / By the word of your lips I have kept from the paths of the violent one.
But I want to focus instead on verse 15. “I in righteousness will behold your face. / I will be satisfied when I awake with your likeness.” This is, first, the Christian hope of seeing the face of Christ, first in the mirror of the Word and then in glory. It is, in the second place, the certainty of being transfigured to be like him by the power of his Spirit in us. Two New Testament verses give particular expression to it.
But we all, face unveiled, beholding in a mirror the glory of the Lord, into the same image are being transfigured from glory unto glory just as from the Lord the Spirit (2 Cor. 3:18).
Beloved, now we are children of God; and it has not yet been manifested what we will be. But we know that when he is manifested we will be like him because we will see him as he is (1 John 3:2).
David, who wrote this psalm, saw the face of the ascended Christ from the perspective of one already in heaven. We will see the same face when we go to heaven to be with him, and then all the glory that belongs to him will pour into us and we will know what Paul meant when he said, “To live is Christ, to die is gain.”