Some people say that a Christian should never pray for forgiveness because his sins are all forgiven in Christ. It is finished in the cross and no more forgiveness is possible. There is truth in this. But to me it smacks of artificiality. At least we should say, "Father, I have sinned and am not worthy to be called your child. Please apply the blood of Christ to this my sin and count it among those which you forgave when he died for me on the cross." But if that is too complex, I am sure the Father would gladly receive the words, "Forgive us our sins as we forgive those who sin against us."
John Piper