Commenting on Philippians 3.7 (Yet whatever gains I had, these I have come to regard as loss because of Christ [NRSV]), Karl Barth writes well.
“To repent – one surely turns here involuntarily to this concept – does not mean to be liberalized, to become indifferent to what we formerly were, to the former objects of our devotion and the former conduct of our lives, but to be horrified by it all. Not realizing that it means nothing but that it means evil. Spinoza does not become a Reformer, but Luther does. The Pharisee Gamaliel does not become an apostle, but the Pharisee Saul does.”
This repentance means realizing that
“The heights on which I stood are abysmal. The assurance in which I lived is lostness; the light I had, darkness. It is not that nil takes the place of the plus, but the plus itself changes to a minus.”
Quoted from Barth’s Epistle to the Philippians (Louisville: WJK, 2002), 97.