John 14:26, “But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things and bring to your remembrance all things that I said to you.”
Those who desire to teach new doctrine, therefore, have no interest in being Christian. All that the Lord has willed us to know, the Holy Spirit has taught Christ’s chosen apostles. Thus, when our ears itch and we desire to hear and invent new doctrine apart from Scripture (2 Timothy 4:3), let us not supplant God’s Word with new, contrary doctrine. For anything opposed to Scripture is not from God. If we hear a new teaching, we ought to test it against Scripture to see if it is holy or wretched (1 John 4:1-3; Galatians 1:8).
Thus, let us say with Luther, “If I see that [a doctrine] agrees with the words of Christ, I consider it true and good. But if it deviates from them and presents something else, I declare: ‘You are not the Holy Spirit; you are the Devil! For the true Spirit comes in the name of no one else than Christ, and He teaches nothing but what Christ said'” (Sermons on the Gospel of John, Chapters 14-16, LW 24:176-177).