"... John now turns abruptly to a serious threat ... The word translated warn means “witness” or “testify” (verses 16, 20; 1:2); Hughes suggests that it is the obverse of “blessed” (1990:240: 1:3; 22:7). The plagues are the bowl judgments. Hence, the punishment fits the offense; the person who tampers with the prophecy receives the judgment of the same prophecy. This warning has roots in a variety of biblical passages (Deuteronomy 4:2; 12:32; 29:19-20; Proverbs 30:6; Galatians 1:6-9; 1 Enoch 104:10; 2 Enoch 48:7-8). Moreover, in days before the printing press and copyright laws, it was common to end a letter with a warning against tampering with its message. The problem is illustrated by Tertullian, who accused Valentinus of perverting his text with additions and alterations (Against Heresies 38) ..."