The Gospel is not a complex riddle to be solved. It isn’t an answer that can only be found after going to college and taking a class in Advanced Physics. It isn’t a hidden knowledge found only by a quest to a remote mountaintop. It is this, believe on the Lord Jesus and you will be saved.
This baptism is a sign of that Gospel promise. The water is like a crown, which shows everyone that this one belongs to the promise of Jesus. Jesus promised that those who believe in Him will be saved from their sins and receive everlasting life, because they belong to Him. This is what is confessed when anyone is baptized: they belong to Christ, and Christ’s promise belongs to them.