To a Christian, the God of the Old Testament, named Jehovah was Jesus Christ himself.
Exodus 6:2-3
2 And God spake unto Moses, and said unto him, I am the Lord:
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.
It was Jehovah that Moses spoke to face to face who is also known as God Almighty and the Great I AM.
Exodus 3:14
14 And God said unto Moses, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you.
The Apostle Paul tells us that the God of Moses was none other than Jesus Christ:
1 Corinthians 10:1–4
1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea;
2 And were all baptized unto Moses in the cloud and in the sea;
3 And did all eat the same spiritual meat;
4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ.
Jesus himself professes that he was I AM:
John 8:58
58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
So it stands that if Jesus was the God of the Old Testament who was known as the Great I AM or Jehovah, then the God who created this earth was Jesus Christ and the God whom the posterity of Adam and Eve worshipped was Jesus Christ. The God who gave Moses the ten commandments was Jesus Christ. True he did not go by the title Jesus or Christ at the time, but he was their God and they worshipped him. Thus they were Christians from the very beginning.
John 1:1-3, 14
1 In the beginning was the Word, and the Word was with God, and the Word was God.
2 The same was in the beginning with God.
3 All things were made by him; and without him was not any thing made that was made.
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.
Jesus reveals that he is the God of the Old Testament in the following verse where he refers back to the Old Testament God (himself) as saying we are gods in Psalms 82:6
John 10:34
34 Jesus answered them, Is it not written in your law, I said, Ye are gods?
Judaism is nothing more than a belief in the Law of Moses. Judah was a son of Jacob (aka Israel), who was the son of Isaac, who was the son of Abraham. After the lesser law of Moses was instituted through Moses because of the hardness of the hearts of the children of Israel, this following of the Law of Moses continued until the time of Jesus and John the Baptist. Jesus came to fulfill the law of Moses to a higher law. Judah and other tribes of Israel that remained followed this law and did not understand that it was to be fulfilled when their very God himself would come among them in the flesh and take upon himself the sins of the world. The Jews rejected Jehovah (Jesus) and to this day continue to do so. Jesus is the first and the last and his ways were had from the beginning. He is the true God of Israel.
The difference between Judaism today and Christianity is that the Jews outright reject Jesus as the Savior. Christians believe that Jesus is the God of the Old Testament and is the God of the Jews and has been since the beginning. Thus Christianity was the first and will be the last as Jesus is the true God of both the Christians and the Jews.
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