18 For those who are such do not serve our Lord Jesus Christ, but their own belly, and by smooth words and flattering speech deceive the hearts of the simple.
Paul now gave the reason to avoid false teachers.
those who are such do not serve our Lord Jesus Christ,
False teachers do not and cannot serve our Lord Jesus Christ, although they probably deem themselves to be Christians.
but their own belly [base appetites],
These false teachers not only do not serve our Lord but they serve themselves and are in Christianity for selfish reasons. They serve themselves with their doctrine.
Jude 16 These are grumblers, complainers, walking according to their own lusts; and they mouth great swelling words, flattering people to gain advantage.
and by smooth words and flattering speech
The methodology of these deceivers is to use deception by smooth words and flattery in order to proselytize others. They have an apparently plausible way of putting forth their false teachings. Often people not taught in the Word will fall for teachers like this. Only mature people grounded in the Word can sort it out.
deceive the hearts of the simple [unsuspecting].
“Deceive” is a very strong term. Second Corinthians 11:3 uses this Greek word for Satan’s deception of Eve.
Many “simple” people today fall for the false teachings of some television preachers. There is a naiveté and simplicity about them. They are deceived through the false promises of these phony preachers. False teachers fool them easily by persuasive talk. This is the main reason naive Christians fall for cultic thinking.
It takes mature people to sort out false teaching.
False teachers are sophisticated, even witty and urbane. They are no unpleasant oafs. There is a danger in influencing by eloquence rather than by objective facts of the Word of God. The Bible never commends the simple-mindedness of the gullible. It expects believers to use shrewdness about what we know about the Bible. A purely emotional believer is a menace to the church. Emotion does not have content; it takes content to have a proper criterion for truth. Emotion cannot provide character, truth, or application of truth to experience.