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.
Jude 1:22
And you should show pity toward some people because they have doubts [regarding spiritual matters];
Revelation 2:2
I know about your deeds, your hard work and your perseverance. [I know] that you cannot tolerate wicked men and that you have tested those who call themselves apostles, but really are not, and found them to be liars.