There's an old chorus that, despite its failings, has an incredible verse. So incredible is this verse that I would venture to say that it's one of the greatest lines of any worship song I know:
He is all my righteouness; I stand complete in Him
What incredible truth! Jesus is ALL my righteousness. He's not just part of it. He's not just the initial righteousness I need to "get in". He is all my righteousness.
You see, even though we believe in the imputation of Christ's righteousness in justification, we can still sometimes end up falling into the trap of divorcing that from the Christian life. We end up thinking of justification as being for the beginning (sort of like conversion), but then think that we need to go on from there.
Someone expressed it to me recently as God's grace in justification raising the bar. But the bar can't be raised. The only righteousness that God accepts is perfect righteousness. And any attempt of our own at righteousness is far from perfect - just remember what Isaiah had to say about that (Isa. 64:6). That's why we need to be justified in the first place! The bar is already as high as it can go. There's no way we can reach it. But thanks be to God, for He has sent His Son to reach it for us, to unite us to Himself, and to clothe us with His perfect righteousness.