Does a person have to believe the doctrine of the Trinity in order to be saved? Can a person only believe that Jesus is the Lord and Savior and the Son of God, but not necessarily God and be saved? And in the Old Testament do you think it is God the Father only who speaks? If so, did the Jews at that time only know part of God?
Jesus makes the following surprise statement in John 8:24,
Therefore I said to you that you will die in your sins; for unless you believe that I am He, you will die in your sins. John 8:24 (NAS95S)
The Greek words that are translated as “I AM” are EGO EIMI. Note that the word “He” is not in the Greek text. Therefore, the word “He” should be deleted in the passage and the verse should read as follows,
Therefore I said to you that you will die in your sins; for unless you believe that I am, you will die in your sins. John 8:24 (NLT)
Jesus declared that He was and is the “I AM” of the Old Testament (Exodus 3:14). Thirty four verses later Jesus repeated His statement in a different way,
Jesus said to them, “Truly, truly, I say to you, before Abraham was born, I am.” John 8:58 (NAS95S)
The Greek words that are translated as “I AM” in verse 58 are again EGO EIMI. Verse 58 reveals that Jesus taught that He was God. The context of John 7 and 8 also confirms that Jesus declared that He was God. So when Jesus said in the first passage that one must believe that “I AM,” He was declaring that we must believe that Jesus is God in order to be a Christian – to go to heaven.
Regarding your last question, the answer is that God the Father and Jesus Christ both speak in the the Old Testament. Since most individuals who are familiar with the Bible believe that God the Father or Jehovah speaks in the Old Testament, we will limit this discussion to the presence of Jesus Christ in the Old Testament. Jesus Christ appears as the angel of the Lord in the Old Testament (Gen. 16:11-17; 18:1-33; 22:11-15; 32:28-30; Ex. 13:21-22; 24:9-11; Judges 12:21-22, and Isaiah 6:1, for example. One of the most remarkable occurrences of Jesus Christ as the angel of the Lord occurs in Exodus 3:14.
Now Moses was pasturing the flock of Jethro his father-in-law, the priest of Midian; and he led the flock to the west side of the wilderness and came to Horeb, the mountain of God. The angel of the LORD appeared to him in a blazing fire from the midst of a bush; and he looked, and behold, the bush was burning with fire, yet the bush was not consumed. So Moses said, “I must turn aside now and see this marvelous sight, why the bush is not burned up.”When the LORD saw that he turned aside to look, God called to him from the midst of the bush and said, “Moses, Moses!” And he said, “Here I am.” Then He said, “Do not come near here; remove your sandals from your feet, for the place on which you are standing is holy ground.” He said also, I am the God of your father, the God of Abraham, the God of Isaac, and the God of Jacob.” Then Moses hid his face, for he was afraid to look at God. . . . God said to Moses, “I AM WHO I AM”; and He said, “Thus you shall say to the sons of Israel, ‘I AM has sent me to you.'” Exod 3:1-6, 14 (NAS95S)
Here we are told that Moses discovers a burning bush in the wilderness and the angel of the Lord speaks to him. Later we are told that God was speaking to Moses. In verses 14 we are told that the angel of the Lord is the great “I AM.” The angel of the Lord was Jesus Christ. Since the angel of the Lord appears on other occasions throughout the Old Testament we discover that Jesus spoke in the Old Testament.
We would encourage you to listen to a two part study titled, “The Trinity Mystery.”