Have mercy on me, O God,
according to your steadfast love;
According to your abundant mercy,
blot out my transgressions.
Against you, you alone, have I sinned,
and done that which is evil in your sight,
So that you are justified in your sentence,
and blameless in your judgement.
Create in me a clean heart, O God,
and put a new and right spirit within me.
Cast me not away from your presence,
and take not your Holy Spirit from me.
O Lord, open my lips,
and my mouth shall proclaim your praise.
Psalm 51 
Today we begin Lent, with ashes and penitence; we undertake practices that will, with the help of the Holy Spirit, turn us back to God. Today I acknowledge that, however satisfactory I think my Christian life is, I still need God to give me a clean heart, and a new and right (or steadfast, as some translations have it) spirit within me.
Psalm 51 is a psalm of David, the one that dates from his famous fall: his sin with Bathsheba and the murder of her husband, Uriah. I can look back on my life since last Easter and see nothing quite so vicious in my own life. And yet–I know my transgressions, I know the dark and cold places in my own heart. I know that ‘I have greatly sinned, in thought and word, in what I have done, and in what I have failed to do.’ My inattention to the Holy Spirit bears fruit of impatience and anger, envy and despair and resentment.
And so I pray with David, ‘Cast me not away from your presence, and take not your Holy Spirit from me. Restore unto me the joy of your salvation’. However much penance I might undertake this Lent, however carefully I might scrutinize my conscience, I cannot do what needs to be done for myself. I can only empty myself to welcome the risen Lord, who himself will give the clean heart that will receive him at Easter.