Philippians 3:7

But whatever gain I had, I counted as loss for the sake of Christ. (Philippians 3:7, ESV)

All of those things, all of that effort, all of that sacrifice, all of that diligence, all of that zeal... not simply worthless, but an actual negative when it comes to the cross.

It isn't that the things Paul had done were bad things, though some of them were, it is that this righteousness by the Law actually worked against Paul's faith in the true messiah. That confidence in the life he had lived did no good, but only harm, in the ways of the Cross.

This is quite possibly because the way of the Cross is a way that realizes that we have nothing in and of ourselves to offer to Christ. The way of the Cross is a way that realizes that we cannot obtain, we cannot reach the righteousness that truly saves.