Some who read the Revelation passage would argue that this is talking about new extra-biblical revelation (over against those who believe the gift of knowledge is for today), or those who would use a translation they don’t consider acceptable. For clarification, I am not speaking of cult translations like the Jehovah’s Witness version of the Greek NT and thus their New World Translation or the Book of Mormon. My primary concern would be those who pick a specific translation and lift it (and it only) up above all others.
I contend the main thrust of these passages is the issue of legalism and liberalism.
In Deuteronomy 4:2, God gives clear instruction “You shall not add to the word that I command you, nor take from it, that you may keep the commandments of the LORD your God that I command you.” This addition was unacceptable, primarily because God knew what they needed to have and extras were just that, extra.
Jesus spoke against the Pharisees and their practice of both behaviors in Matthew 23. In verse 4, “They tie up heavy burdens, hard to bear, and lay them on people’s shoulders, but they themselves are not willing to move them with their finger.” This shows the addition of rules not found in God’s law. In verse 23, Jesus declared they had “neglected the weightier matters of the law: justice and mercy and faithfulness. These you ought to have done, without neglecting the others.”
While teaching the people rules, they followed a different path. Thus, teaching legalism (adding to God’s instruction) they practiced liberalism (ignoring God’s instruction).
Proverbs 30:6 warns against adding to the word of God, “Do not add to his words, lest he rebuke you and you be found a liar.”
The last statement on the issue in Scripture is the Revelation passage (22:18-19). My understanding is that this applies specifically to the words of Jesus as found in Revelation. Trying to make it fit all Scripture may preach well, but it is a hermeneutical stretch, in my opinion.
I believe the warning found in the Revelation verses is similar to that of Deuteronomy and Proverbs. Don’t add to what Jesus said, or he will add the plagues; don’t take away from what Jesus said, or your name will be removed from the book of life. That’s a serious warning.
All who would add to the teaching of God as found in Scripture are often labeled legalists and those who would take it away are liberals. Both are guilty of opposing the clear teaching of Scripture. May we be like Paul and walk the middle line between either excess and avoid the problems inherent in either side.