What does Romans 6:17 mean?

Paul has been warning the Christians in Rome not to volunteer to sin. They have been freed from the authority of and slavery to sin. It is not in command of their bodies any longer, thanks to Christ's death for their sin on the cross and their spiritual death with Him when they were saved.

Now Paul changes his tone and acknowledges that his Christian readers in Rome have become obedient from their hearts, meaning that they have sincerely committed themselves to obeying. Obeying what? They have become obedient to a standard—or pattern—of teaching. This phrase uses the Greek term typon, used of the mark made by a hammer, or the surface used to imprint an image. This is a set of doctrines to which these Roman Christian have been entrusted, or "committed," or "handed over."

What Paul is describing is this: The Roman Christians came to faith in Christ at some point. They became Christians. In that moment, sin lost its true power over them. They stopped being slaves to sin. God—and, in a sense, those who led these new believers to Christ—handed them over to the teaching of God's truth by their leaders. The new Christians became obedient to that teaching instead of living as slaves to sin.

This is the pattern of the church, God's plan for Christians since the very beginning. Acts 2:42 describes the earliest Christians as being devoted to the apostles' teaching. Even today, those who trust in Christ are meant to devote themselves to obeying the teachings of God's Word.