In this passage in Matthew 16, God is showing us that if we are not careful, if we are not completely submissive to Christ and His word, allowing Christ Himself to determine what is right and wrong in our understanding, in our theology and doctrine, and in the kinds of things that we say and do, then we can very easily stumble into the most serious sins we can imagine, even opposing Christ and the gospel. Those of us who are genuine believers don-t want that- God warns us of the very real danger of standing in opposition to Him. And He shows us here what the trap is so that we can avoid that trap and avoid making the same horrific mistake that Peter did.