We could picture some writer seeking to insert a paragraph somewhere within the body of the Lord’s revelation to John and understand that the Lord disallows such effort. More sinister would be for someone to accept the book as it is, but in his teaching add some extra-biblical material or reinterpret (parts of) God’s revelation in lines contradicting God’s clear intent. Similar language as John uses in the present passage occurs in Deuteronomy 4:1–2 and Deuteronomy 12:32. The Deuteronomy 4 passage introduces a recollection of what God did at Baal Peor to those who followed the deception of Balaam (see also Numbers 25:1–9). The Deuteronomy 12 passage introduces the paragraph outlining how the Israelites were to respond to a prophet or dreamer in their midst who suggests that Israel should serve other gods (Deuteronomy 13:1–5). In both cases, instruction is given that goes beyond what God has instructed (and so implicitly also takes away from God’s revelation). Both of those passages relate to some form of idolatry.
18 I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book,