The God of the Old Testament

Jacob, Moses and Hagar saw God:
"So Jacob called the name of the place Peniel, saying, "For I have seen God face to face, and yet my life is preserved." (Genesis 32:30 RSV)

"Then Moses and Aaron, Nadab, and Abihu, and seventy of the elders of Israel went up, and they saw the God of Israel; and there was under His feet as it were a pavement of sapphire stone, like the very heaven for clearness. And He did not lay His hand on the chief men of the people of Israel; they beheld God, and ate and drank." (Exodus 24:9-11 RSV)

"So she called The Name of The Lord who spoke to her, "Thou art a God of seeing"; for she said, "Have I really seen God and remained alive after seeing Him?" Therefore the well was called Beer-lahairoi; it lies between Kadesh and Bered. And Hagar bore Abram a son; and Abram called the name of his son, whom Hagar bore, Ishmael." (Genesis 16:13-15 RSV)

But, no man or woman has ever seen God (The Father):
"For the Law was given through Moses; grace and truth came through Jesus Christ. No one has ever seen God; the only Son, Who is in the bosom of The Father, He has made Him known." (John 1:17-18 RSV)

The "God of the Old Testament" was the Logos of God, Who was later born as Jesus Christ:
"I want you to know, brethren, that our fathers were all under the cloud, and all passed through the sea, and all were baptized into Moses in the cloud and in the sea, and all ate the same supernatural food and all drank the same supernatural drink. For they drank from the supernatural Rock which followed them, and the Rock was Christ." (1 Corinthians 10:1-4 RSV)

The Logos God
It was the Logos, or Word, Who was later born as the human Jesus Christ, that created and spoke to Adam, for "all things were made through Him, and without Him was not anything made that was made" (John 1:3 RSV). When Adam sinned, when he chose Satan's way, mankind was cut off from God (The Father) until Christ, "the second Adam" (1 Corinthians 15:45-49) paid the penalty for all sin.

Despite the obvious implication of the "us" and "our" of Genesis 1:26 (that some have interpretted to mean that God was merely speaking to Himself figuratively, such as when a human king would say, "We have decided," when speaking only of himself), the existence of The Father was unknown until Jesus Christ revealed Him: "All things have been delivered to Me by My Father; and no one knows Who the Son is except the Father, or who the Father is except the Son and any one to whom the Son chooses to reveal Him." (Luke 10:22 RSV)

Although surprising to some, mankind's knowledge about God increased in stages through Bible History according to God's will, at times of His choosing e.g. "And God said to Moses, "I am The Lord. I appeared to Abraham, to Isaac, and to Jacob, as God Almighty, but by My Name The Lord I did not make Myself known to them." (Exodus 6:2-3 RSV)

It was the Logos/Jesus Who revealed The Father's existence when He revealed His Own existence. For that, Jesus was fiercely rejected and accused of blasphemy by the religious authorities of His day when He rightfully claimed to be the Son of God because they were not aware that God exists as The Father and The Word/Logos. Before He was born as a human, no one knew that there was a Logos, so they then, as many still do now, assumed that the "God of the Old Testament" was The Father.