In Deuteronomy 12 we have a definite word on keeping the oneness of God's people. In chapter thirteen, which we will consider in this message, we have an even more definite word—a word concerning apostasy.
What does the word apostasy denote? In the Old Testament, apostasy denotes the giving up of God and the turning away from God to idols. In the New Testament, apostasy denotes the denial of Christ's deity; it refers to not believing that Christ is God incarnated to be a man.
The existence of God is a mystery, and the incarnation of God to become a man named Jesus is an even greater mystery. For this reason it is more difficult for people to believe that God became a man than it is for them to believe that there is a God. As we contact people in our preaching of the gospel, we must first make it clear to them that there is a God. We may use the book of Romans to show them that there is a God. Then we need to tell them that this very God was incarnated to become a man.
Certain so-called Christians—the Modernists—do not believe that Jesus Christ is God. Instead, they believe that the Lord Jesus was merely a man, that He was a teacher, and that He died as a martyr, not to accomplish redemption. They do not confess that Jesus Christ has come in the flesh (1 John 4:2-3). Their belief regarding Christ is heretical, and, by denying that Jesus Christ is both the complete God and a perfect man, they have taken the way of apostasy.
The New Testament deals very strictly with division and with the denial of the teaching that Christ is God incarnated to become a man. "If anyone comes to you and does not bring this teaching, do not receive him into your house, and do not say to him, Rejoice! For he who says to him, Rejoice, shares in his evil works" (2 John 10-11). The teaching which the Apostle John refers to here is the teaching that God came through incarnation to be a man. In his gospel this same John says, "In the beginning was the Word, and the Word was with God, and the Word was God....And the Word became flesh and tabernacled among us...full of grace and reality" (John 1:1, 14). In his second epistle John shows us that, even at his time, some so-called believers did not confess that Jesus Christ the Lord had come in the flesh. John goes on to tell us that we should not receive nor even greet those who do not bring the teaching that Jesus Christ is God incarnated to be a man. Instead of speaking to those who do not bring this teaching, we should turn away from them.