“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” (I John 4:2 NIV).
Everything we believe as true Christians is tied to what we believe about the Person of Jesus Christ. Jesus is the cornerstone on which all of our doctrine rests. Therefore we can recognize true believers by what they believe about Jesus Christ. We can also recognize imposters by what they don’t believe about Jesus Christ.
This does not mean, however, that everyone who claims to believe that Jesus is the Christ is a true believer. Of course false teachers will lie about what they believe while denying Jesus in subtle ways. Train yourself to detect subtle, sneaky attacks against what the Bible says about Jesus.