Belief is an important part of Christianity, but so is disbelief.
The Bible teaches the importance of belief (or faith) in several places. According to 1 John 3:23, belief is a divine requirement: “And this is [God’s] command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us.” According to Ephesians 2:8-9, faith is instrumental in our salvation: “For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God—not by works, so that no one can boast.” Indeed, “without faith,” as we read in Hebrews 11:6, “it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who diligently seek him.” There simply is no Christianity without belief. That is why Christians are also called believers.
But by the same token, there is no Christianity without disbelief. There are some things about which Christians are outright skeptics. First John 4:1-3 is the foundational verse of Christian disbelief:
Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
The gift of prophecy was very active in the New Testament church, as it is to a certain extent even today. In the biblical understanding of prophecy, the words of a prophet are inspired by the Holy Spirit. According to 2 Peter 1:21: “prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.” But what happens when two people, claiming to speak under the alleged inspiration of the Holy Spirit, make contradictory truth claims? Obviously, since God is a logical God, one of them is not a true prophet; he or she is not telling the divinely inspired truth. But which one?
First John 4:1-3 provides an answer to this question. It teaches us to “test the spirits,” that is, to use our God-given brains and apply rational criteria for sorting out the truth of a matter. One of the specific rational criteria we must use is Christological in nature: “Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God.” In other words, any supposedly spiritual person must at a very minimum affirm the obvious historical truth about Jesus—that he came “in the flesh.” If the spirit cannot affirm this baseline truth, nothing it says is true.
There are other rational criteria, of course. A prophet cannot speak in logical contradictions. He or she cannot advocate immorality. But according to this passage, if we are going to believe a word of prophecy, it must conform to the historical evidence about Jesus. Otherwise, we are free—nay, required—to disbelieve it.