What does 1 Corinthians 10:8 mean?

Paul has presented two examples of God's discipline on His people Israel for their sinfulness while they lived in the wilderness. A major point of these examples is to remind Christians that God takes sin seriously, even in those who are saved. Sin brings consequences, and that can include discipline from God. Here, Paul provides a third instance.

The people of Israel engaged, in large numbers, in sexual immorality with the "daughters of Moab" as part of the worship of Moab's gods, including Baal (Numbers 25:1–3). Idol worship in Scripture is often closely related to sexual immorality of all kinds. In some cases, this is literal, as sex acts were sometimes used as part of the worship of false gods. It was true for Israel, and it was true in the worship of idols in Corinth.

Once again, the Lord stepped in to discipline Israel for this sin. Before it was all over, 24,000 had been killed by another plague from the Lord (Numbers 25:9). Paul's count of 23,000 may only include those killed on a single day—a detail specified here, but not in the Old Testament's count of 24,000. Some number of those afflicted may have lingered long enough to die on some later date.

The Christians in Corinth, too, faced ongoing temptation to participate in sexual immorality, whether connected to idol worship or not. Such immorality was so common in the culture of their day that it required faith to even understand such actions as sinful and offensive to God. Paul, though, is clear, that the Lord may discipline those who rebel in this way. The Corinthians "must" not indulge in sexual immorality while assuming God will not step in to correct their course—or end it, entirely.