In his Epistle to the Romans, the Apostle Paul warned the early Christians to watch out for false teachers:

NKJ Romans 16:17-18 "Now I urge you, brethren, note those who cause divisions and offenses, contrary to the doctrine which you learned, and avoid them. 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."

I recently ran across a video of John Piper explaining Paul's meaning in this passage, in particular verse 18, and I thought the blog's readers might appreciate it as much as I did.