18 Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have appeared; from this we know that it is the last hour. 19 They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us. 20 But you have an anointing from the Holy One, and you all know. 21 I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. 22 Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist , the one who denies the Father and the Son. 23 Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. 24 As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father. 1 John 2:18-24
What makes Christianity different from other religions, apart from the object of our faith being the one true and living God, is knowing in advance the end of the story. The Apostle John had a big role in this, having penned The Revelation while exiled on the isle of Patmos in his final days. The antagonist in this story is the antichrist, the ultimate Christ-denier and consummate liar.
But until “the antichrist” is revealed–not to the church, because we will be long gone–we must contend with a proliferation of many junior antichrists, who also in perhaps subtle ways deny Christ and who lie through their teeth. John says these charlatans will eventually leave the church when ferreted out, so they are the most dangerous when still under cover. As Satan disguises himself as an “angel of light” we must always be wary of those who teach us–in a polite way–double-checking what they tell us, as did the Bereans (Acts 17:11).
The proof is always in the pudding. If a teacher’s error appears to involve disavowing the Christ of the scriptures, he must be exposed. If he accepts correction and direction and stays in the body, he will be found to possess sound judgment and a certain “anointing from the Holy One,” i.e. teachability by the Holy Spirit. If he leaves, certainly what is suspected–that he is a false teacher–is validated.
Now there are doctrinal differences–some petty–that may lead to eventual separation, as various denominations and new “hip” movements may only see one side of the coin or the other. But the true test of sound teaching is always the absolute unambiguous belief that Jesus is the Christ, the son of the living God, the one and only route to salvation.
If this core belief is absent, count on this person hitting the exit sign when pressed.