Have mercy on me, O God,
according to your steadfast love (v. 1).
We must remember that the psalmist is writing after the commission of a horrific crime. This is written after he had slept with Bathsheba and arranged for her husband to die in battle. So it is all the more poignant that David connects the mercy he wishes to receive with some abiding element of God's character. We all know the refrain: his steadfast love endures forever, repeated ad infinitum in Ps 136. The idea here is that God is of a certain character, disposed to forgive even the desperately wicked of their sins.
God is merciful and his love is steadfast, even in the face of our sins. This is obvious, theology 101-type stuff, but oftentimes we can forget it. In our guilt and in our sense of shame for what we have done, we sooner wallow in depression and self-loathing (or worse, we try to justify ourselves) rather than to come before God with repentance and to receive forgiveness just as quickly. David's point in the petition is this: Lord, I need you to forgive me and be merciful, and I come to you because I know this is who you are.
Wash me thoroughly from my iniquity,
and cleanse me from my sin (v. 2).
Only God can cleanse us of our sins; we cannot accomplish this by ourselves. Such is the desperately fallen state of man: he gets himself stuck in the mire and swamp of sin, and he needs to look outside of himself if he is going to have any hope whatsoever. Thankfully, God is such that he always stretches out his hand to us, and more than that, sometimes he redirects our attention to notice it. But he can't make us grab his hand, or at the very least, he can't force us to accept when he would pull us out. Some struggle against this salvific act of God and reject his attempts to save him.
David is not like that. He recognizes the desperation of his state, and calls out to God to receive mercy. He is aware that only God can make him clean, after he has defiled himself so utterly with these grave sins of adultery and murder. With this knowledge, he brings his petitions to God, rather than trying to make things right himself.
Create in me a clean heart, O God,
and put a new and right spirit within me (v. 10).
When we turn from sin and orient ourselves towards the Lord, we must become new persons. In the New Testament, Paul especially talks about putting off the old self and taking on the new self, modeled after the example and image of Christ in righteousness (Rom 6; Eph 4; Col 3). This change has to take place has to take place on the inside, however, just as much as on the outside. It is not enough merely to abstain from the sins of yesterday, as if limiting yourself to one instance of adultery and murder were enough to make your righteous. Rather, you must take on a new heart: the previous heart of stone has to be replaced with one of flesh.
In the New Testament, we see that this takes place through the activity of the Holy Spirit, which convicts a person of her sins, teaches her the truth, and leads her to trust and love and hope in God. Thus David says: do not take your holy spirit from me, knowing that this kind of interior change has to be accomplished by God himself rather than by us. We must merely open ourselves up to it and pray for it.
Then I will teach transgressors your ways,
and sinners will return to you (v. 13).
There is power, I think, in the testimony of a person whose life has been changed. I have close friends who have turned from sin and darkness to the Lord in very drastic and striking ways, and the testimony they offer to the continuing power and activity of God in the salvation of sinners is very impressive.
To some extent, I suppose that David's words here offer an example for us. When the Lord turns us from our sins, we should not assume a kind of solitary, self-centered existence that no longer has any regard for others. Rather, knowing the salvation of God personally, we ought to bring others to experience it as well. I think that this the essence of the missionary spirit, and present in the heart of every missionary: the desire to see others enjoying the same salvation that you have come to know in Jesus Christ. So David says that, when God forgives him and justifies him, he will teach transgressors the way of God and sinners will return to God.
Notice, too, that he says that sinners will return to God. This is because living in fellowship with God, in obedience to his commandments, is not unnatural to human persons but, on the contrary, it is the way we were created to live. Before the descent of humankind into sin, we were created to live in love and unity with God and with one another. A turn to God is therefore a return to what is normal and natural. A person who has discovered the joy and beauty of life with God has really discovered her true self; she has discovered who she truly is, and that previous life in sin and enmity with God is looked upon as falsehood and lies and ignorance.