"The determining factor in our relationship with God is not our past or present, but Christ's past and present.
'How then does it work?' It works like this. God accepts this righteousness of Christ, this perfect righteousness face to face with the Law, which He honored in every respect. He has kept it and given obedience to it [through his perfect life], and he has borne its penalty [through his death]. The Law is fully satisfied. God's way of salvation, says Paul, is that. He gives to us the righteousness of Christ. If we have seen our need and go to God and confess it, God will give us his own Son's righteousness. He imputes Christ's righteousness to us, who believe in Him, and regards us as righteous, and declares and pronounces us to be righteous in Him. That is the way of salvation, the Christian way of salvation...
To make it quite practical let me say that there is a very simple way of testing yourself to know whether you believe that... [After] I have explained the way of justification...to them, then I say: 'Well, then, you are now ready to say that you are a Christian?' And they hesitate. And I know they have not understood. Then I say: 'What is the matter, why are you hesitating?' And they say: 'I do not feel that I am good enough.' At once I know that in a sense I have been wasting my breath. They are still thinking in terms of themselves; their idea still is that they have to make themselves good enough to be a Christian, good enough to be accepted with Christ. They have to do it! 'I am not good enough.' It sounds very modest, but it is the lie of the devil, it is a denial of the faith... The essence of the Christian faith is to say that He is good enough and that I am in Him!
As long as you go on thinking about yourself and saying: 'Ah, yes, I would like to, but I am not good enough; I am a sinner, a great sinner,' you are denying God and you will never be happy. You will continue to be cast down and disquieted. You will think you are better at times and then again you will find that you are not as good as you thought you were... How can I put this plainly? It does not matter if you have almost entered into the depths of hell, if you are guilty of murder as well as every other vile sin, it does not matter from the standpoint of being justified before God. You are no more hopeless than the most respectable...person in the world. Do you believe that?"
D.M. Lloyd-Jones, Spiritual Depression