The Corinthians accuse Paul of being reprobate. They demand proof of his ministry. They request evidence of Christ in him. Paul's response is to have them examine themselves. Do they know who they are? The answer to that question should cause them to be honest and to stop resisting rebuke and correction.
Regardless of men, or their actions, the truth is that preaching of the cross is the power of God unto salvation. Believing on His work on your behalf is the only way we can receive Christ. If Christ is in you today, you are saved. If He is not, you are reprobate.