Jesus Christ
Hearken, O ye people of my church, saith the Lord your God, and hear the word of the Lord concerning you— Doctrine and Covenants 133:1
The whole section is spoken by the Lord Jesus Christ to 'my church' — the Church that bears His name. He is the one calling His people to prepare for His coming.
How we know: the chapter heading names Him
- the same title in D&C 1:1 — The Preface opens with the same 'Hearken, O ye people of my church'; this Appendix closes the book in His voice.
