From a sermon of Martin Luther for Christmas Day, reprinted in Sermons of Martin Luther, vol. I, pp. 181-83 (Baker Book House, Grand Rapids, MI, USA: 1983)
“And the Word was with God.”
21. … But here he clearly distinguishes the persons, so that the Word is a different person than God with whom it was. This passage of John does not allow the interpretation that God had been alone, because it says that something had been with God, namely, the Word. If he had been alone, why would he need to say: The Word was with God? To have something with him, is not to be alone or by himself. It should not be forgotten that the Evangelist strongly emphasizes the little word “with.” For he repeats it, and clearly expresses the difference in persons to gainsay natural reason and future heretics. For while natural reason can understand that there is but one God, and many passages of Scripture substantiate it, and this is also true, yet the Scriptures also strongly oppose the idea that this same God is only one person.
22. Thus arose the heresy of Sabellius, who said: The Father, Son, and Holy Ghost are only one person. And again Arius, although he admitted that the Word was with God would not admit that he was true God. The former confesses and teaches too great a simplicity of God; the latter too great a multiplicity. The former mingles the persons; the latter separates the natures. But the true Christian faith takes the mean, teaches and confesses separate persons and an undivided nature. The Father is a different person from the Son, but he is not another God. Natural reason cannot comprehend this; it must be apprehended by faith alone. Natural reason produces error and heresy; faith teaches and maintains the truth; for it clings to the Scriptures, which do not deceive or lie.
“The same was in the beginning with God.”
25. The Word was with God, with God, and yet God was the Word. Thus the Evangelist contends that both assertions are true: God is the Word, and the Word is with God; one nature of divine essence, and yet not one person only. Each person is God complete and entire, in the beginning and eternally. These are the passages upon which our faith is founded and to which we must hold fast. For it is entirely above reason that there should be three persons and each one perfect and true God, and yet not three Gods but one God.
26. The Scholastics have argued much pro and con with their numerous subtleties, to make this doctrine comprehensible. But if you do not wish to become entangled in the meshes of the enemy, ignore their cunning, arrogance, and subtleties, and hold to these divine words. Press into them and remain in them, like a hare in a rocky crevice. If you come out and deign to listen to human talk, the enemy will lead you on and overcome you, so that you will at last not know where reason, faith, God, or even yourself are.
27. Believe me, as one who has experienced and tried it, and who does not talk into an empty barrel; the Scriptures are not given us for naught. If reason could have kept on the right road, the Scriptures would not have been given us. Take an example in the case of Arius and Sabellius. Had they clung to the Scriptures and disregarded reason, they would not have originated so much trouble in the church. And our Scholastics might have been Christians, had they ceased fooling with their subtleties and had clung to the Scriptures.