The issue is straightforward. If the Corinthians cannot see that Jesus Christ is living among them, they have failed the test of true faith (literally are disproved). There is a wordplay here: They wanted "proof" of Paul's authority as an apostle, but Paul tells them to test themselves (or prove yourselves). If Christ is not present, it would show they are not true Christians. However, Paul has shown that he has passed the test of apostolic authority (literally not been disproved).