2 Corinthians 11:13-15
For such men are false apostles; they are deceitful workers, who disguise themselves as [true] apostles of Christ.
Matthew 7:15
"Look out for deceiving prophets, who approach you wearing sheep's clothing, but who are [actually] starving wolves in their hearts.
Philippians 3:2
Watch out for the "dogs!" [Note: "Dogs" were viewed with disgust by the ancient Jews (Deut. 23:18; II Kings 8:13; Rev. 22:15), so the term is used here derogatorily for Jewish Christians who were trying to impose on the church the requirements of the law of Moses]. Watch out for the evil workers! Watch out for those who mutilate the body meaninglessly! [Note: Paul here alludes, in a derogatory wordplay, to the Jewish practice of circumcision].
2 Peter 2:1-3
But just as there were false prophets among the people before, there will also be false teachers among you [Christians]. They will privately teach harmful doctrines that destroy people [spiritually], by even denying [that Jesus is] the Master, [and thereby] destroying themselves quickly.
1 John 4:1
Loved ones, do not believe [the message of] every spirit [i.e., every person claiming to be a teacher], but test the spirits [i.e., require proof that they and their message are genuine] to see whether they represent God [or not], because many false prophets have begun circulating throughout the world.