2 John 1:9-10, this is not a small warning, John is not saying be careful, he is saying, do not even welcome them, do not invite them in, do not support them, do not align yourself with them in any way. False doctrine is not just wrong, it is deadly, because false doctrine works, the enemy does not destroy people by removing the gospel, they destroys them by distorting it, by taking the truth and mixing it with just enough lies to make it lethal. 2 Corinthians 11:14-15, Satan disguises himself as an angel of light, so it is no surprise if his servants also disguise themselves as servants of righteous. False teachers do not look like false teachers, they look righteous, they look good, they use the name of Jesus, they quote scripture, but they twisted just enough to lead people astray.