I say this because many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist. Watch out that you do not lose what we have worked for, but that you may be rewarded fully. Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not take them into your house or welcome them. Anyone who welcomes them shares in their wicked work.
2 John 7-11 (NIV)