In the first chapters of the book of Revelation, the Lord Jesus sent warnings to the churches, and said everyone who reads those warnings should beware. The reason He warns is so that we can judge ourselves so He wouldn't have to judge us and so that we will not be judged with the world. These messages are for Christians to know what matters to the Lord and what to correct in their personal lives. It is an analysis of the letters of the Lord Jesus to His church.
