I warned those who sinned previously and all the others, and I warn them now while absent, as I did when present on my second visit, that if I come again, I will not be lenient—since you desire proof that Christ is speaking in me. He is not weak in dealing with you, but is powerful in you. For he was crucified in weakness, but lives by the power of God. For we are weak in him, but in dealing with you we will live with him by the power of God. Examine yourselves to see whether you are living in the faith. Test yourselves. Do you not realize that Jesus Christ is in you? —unless, indeed, you fail to meet the test! I hope you will find out that we have not failed. (NRSV 2 Corinthians 13:2-6)
I told you before, and foretell you, as if I were present, the second time; and being absent now I write to them which heretofore have sinned, and to all other, that, if I come again, I will not spare: Since ye seek a proof of Christ speaking in me, which to you-ward is not weak, but is mighty in you. For though he was crucified through weakness, yet he liveth by the power of God. For we also are weak in him, but we shall live with him by the power of God toward you. Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? But I trust that ye shall know that we are not reprobates. (KJV 2 Corinthians 13:2-6)
Holy, serious accusation, Paul! The Corinthians need proof of Paul's authority, and he tells them just to take a look at themselves. If they really love Christ, they'll see that he's the real deal. And if they come to the opposite conclusion… well, they must not be very good Christians. Ouch.