God had been seen in the O.T. appearing to men that did not die when they had seen Him.
The Lord appeared to Abraham face to face.
Genesis 12:7 And the Lord appeared unto Abram, and said, Unto thy seed will I give this land: and there builded he an altar unto the Lord, who appeared unto him.
Genesis 17:1 And when Abram was ninety years old and nine, the Lord appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.
Genesis 18:1 And the Lord appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; 2 And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground,
3 And said, My Lord, if now I have found favour in thy sight, pass not away, I pray thee, from thy servant:
The Lord appeared to Isaac face to face.
Genesis 26:1 And there was a famine in the land, beside the first famine that was in the days of Abraham. And Isaac went unto Abimelech king of the Philistines unto Gerar. 2 And the Lord appeared unto him, and said, Go not down into Egypt; dwell in the land which I shall tell thee of: 3 Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father;
The Lord appeared to Jacob face to face.
Genesis 32:24 And Jacob was left alone; and there wrestled a man with him until the breaking of the day. 25 And when he saw that he prevailed not against him, he touched the hollow of his thigh; and the hollow of Jacob's thigh was out of joint, as he wrestled with him. 26 And he said, Let me go, for the day breaketh. And he said, I will not let thee go, except thou bless me. 27 And he said unto him, What is thy name? And he said, Jacob. 28 And he said, Thy name shall be called no more Jacob, but Israel: for as a prince hast thou power with God and with men, and hast prevailed. 29 And Jacob asked him, and said, Tell me, I pray thee, thy name. And he said, Wherefore is it that thou dost ask after my name? And he blessed him there. 30 And Jacob called the name of the place Peniel: for I have seen God face to face, and my life is preserved.
So this verse below is confirmed.
Exodus 6:3 And I appeared unto Abraham, unto Isaac, and unto Jacob, by the name of God Almighty, but by my name Jehovah was I not known to them.
So Who is the God that men had seen? Was it the Father?
John 1:18 No man hath seen God at any time, the only begotten Son, which is in the bosom of the Father, he hath declared him.
John 6:46 Not that any man hath seen the Father, save he which is of God, he hath seen the Father.
So pretty much the two verses are testifying that no man had seen God the Father, save Jesus, Who is God.
So Who was the God that men had seen in the O.T.?
John 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me. 40 And ye will not come to me, that ye might have life.....46 For had ye believed Moses, ye would have believed me; for he wrote of me. 47 But if ye believe not his writings, how shall ye believe my words?
John 8:56 Your father Abraham rejoiced to see my day: and he saw it, and was glad. 57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham? 58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am. 59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
So the Son of God was the God that men had seen before His incarnation as the Son of Man.
John 1:10 He was in the world, and the world was made by him, and the world knew him not. 11 He came unto his own, and his own received him not. 12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: 13 Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. 14 And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.