What does 1 Corinthians 15:33 mean?

This passage urges those in Corinth who deny Christian resurrection to accept what is true. Now Paul warns all of them not to be deceived by those who don't believe in the resurrection. He may be quoting a popular phrase of the day when he says "bad company ruins good morals." In so far as it's being used here, that's a sentiment Paul is endorsing. There are distinct, spiritual dangers to associating with those who are out of sync with God.

In chapter 5, Paul warned the Corinthians not to associate with other believers who practice various kinds of immorality (1 Corinthians 5:11). He echoes that instruction again here in the form of this proverb. Spending time with people who continually proclaim falsehood—including falsehood about the resurrection of believers—has a way of wearing down even the most faithful follower of Christ. The result of loosening one's grip on right belief inevitably leads to loosening one's moral convictions, as well. It might not result in wholesale apostasy, but it can be damaging.