1 Corinthians 15:9

For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God (KJV)


Then: Paul admits to the Corinthians what they already know. He did persecute the church, and yet this church exists because of Paul's conversion. Still, some had managed to turn away from him to extol the greatness of Peter and Apollos. In the face of a guy telling you, you're right my past is not great, the Corinthians needed to check back in their own past and remember that without Christ no one is righteous.

Now: The human condition can always overwhelm anyone, anytime. Touching the Lord, being grateful, and always remembering, keeps us from being in and/or causing great harm. Paul is an example, even today.