You did not answer my question. Why didn't Peter tell those in Acts 2 that they needed to do nothing else since they already believed that Jesus was the Son of God?
He never told them that they were already saved, and that baptism is simply a sign of their belief. Also, if not getting baptized is an act of disobedience, then your idea of salvation by faith only means that someone can be saved while being disobedient to God. That is in direct contradiction to the teachings of Christ.