What does 1 Corinthians 1:6 mean?

Paul has given thanks, in the previous verses, for God's grace to the Christians in Corinth, including his grace in making them rich in speech and knowledge.

He says now that "the testimony about Christ was confirmed" in them. In other words, Paul sees clear, doubtless evidence that they genuinely believed his teaching. He is assured they have trusted in Christ for their salvation. Their speech and knowledge, among other gifts, showed that they had received God's grace through faith in Christ, just as Paul presented that testimony to them.

Major issues needed to be corrected in the Corinthian church, but Paul was first and foremost grateful for their faith in Christ and God's grace to them. This is a useful piece of information in our understanding of the Christian faith; reminding us that true believers are not immune from spiritual error, and that spiritual mistakes do not prove someone is a non-Christian. Paul makes the Corinthians' position in Christ even more clear in the following verses.