For I testify unto every man that heareth the words of the prophecy of this book. Omit "for." The pronoun "I" is emphatic. Here is the solemn appendix or seal of the veracity of the book, somewhat similar to the prefatory words in Revelation 1:1-3.
This is the fulfilment of the duty laid upon St. John in Revelation 1:1, not an announcement of our Lord himself (cf. the wording of Revelation 1:3). If any man shall add unto these things, God shall add unto him the plagues that are written in this book; cf.
the command in Deuteronomy 4:2, "Ye shall not add unto the word which I command you, neither shall ye diminish from it" (Revised Version). "The plagues that are written in this book" are those of the seals, the trumpets, the vials, the doom of Babylon, etc.
; cf. the command of St. Paul to Timothy (2 Timothy 1:13), and cf. also what is said in 2 Timothy 2:16-18 concerning the heretical teaching of Hymenaeus and Philetus.