Luther on Genesis 35:6-7.
6. And Jacob came to Luz (that is, Bethel), which is in the land of Canaan, he and all the people who were with him,
7. and there he built an altar and called the place El-bethel, because there God had revealed Himself to him when he fled from his brother.
In the Hebrew, however, it is expressly stated: “Because there God (plural) had revealed Himself.” When Scripture speaks of God in the plural, such passages should be carefully noted. Above, in chapter 1:26, it is stated: “Let Us make man in Our image, after Our likeness.” It is likewise stated (Gen. 3:22): “Behold, the man has become like one of Us,” and (Gen. 11:7): “Come, let Us go down, and there confuse their language.” In chapter 18:2–3 we read that Abraham saw three men and said to them: “My Lord, if I have found favor in Thy sight, do not pass by Thy servant, etc.” These passages are truly noteworthy. For they signify that the fathers had the same knowledge and faith concerning the Deity which we teach today. It is foolish to object that the doctrine of the Trinity was nowhere promulgated in the Old Testament, when it is manifest from so many passages that the godly men at that time also concluded that there was a plurality of Persons in the Godhead. But this mystery was not expressed by means of individual letters. This was reserved for the New Testament, where there are very clear testimonies.
Jacob, however, celebrated this revelation in many illustrious sermons. For this reason the altar was erected in memory of this excellent gift, namely, that the Trinity had been revealed to him and the incarnation of Christ and spiritual and bodily promises had been handed down at one and the same time. Nor were those wanting who heard these things listlessly and with heavy ears. But the patriarch received great light and consolation from this source, and since he escaped the danger into which he had been thrown by his sons, he now spent his days without worry in peace and found repose in the promises and divine revelations. But another disaster will soon follow; there is nothing permanent in the life of the saints, and everything is full of varied conflicts, although not without interspersed releases.
Luther’s works, vol. 6: Lectures on Genesis: Chapters 31-37 p 250-252