What does 1 Corinthians 6:17 mean?

Paul is building the case for why it is so important for Christians to "flee from sexual immorality," as he will say in the following verse (1 Corinthians 6:18). He has shown that our bodies are more than just animal carcasses that will die and decay. Our bodies will be resurrected, as Jesus' body was. More, our bodies right now are members of Christ. They are set apart for Him and for His purposes.

This passage has also shown that sex is more than just another mundane human appetite to be satisfied. Sex joins two people together as one, physically and spiritually, which is exactly what God intended for marriage. To have sex with a prostitute, then, Paul has written, is to join Christ with a prostitute, in a sense.

Now Paul adds that Christians, those joined to the Lord, are spiritually united to Him. More specifically, we are one spirit with Him. Paul's point is that what happens to Christ's spirit affects our own. Perhaps surprisingly, what happens in our spirit affects His. That idea suddenly makes the stakes much higher for what we do with our bodies and spirits, including sexual immorality.