Summary: Study 9 - The Trinity is to be seen throughout the Old Testament.
The Old Testament says that God is ONE, but does it ever say that God is MORE than ONE. Indeed it doe. If you study the Old Testament you will see the remarkable fact that, when it uses the Name "God", it uses it in the singular 250 times, but it uses it in the plural 2,000 times! In Genesis chapter 1 verse 1 the literal translation of the Hebrew is, "In the beginning GODS said", and this literal translation is repeated all through the Old Testament in the proportion of 8 to 1. So the Name of God is "GODS"!
The same is true of the Name "LORDS" - in the Old Testament. It is used 30 times in the singlular, but 300 times in the plural. So, when Moses speaks to Israel, he says literally, "Hear, O Israel, the Lord our GODS, the LORDS is ONE". Even when he is telling them that God is ONE, he is telling them that God is MORE THAN ONE.
Even that word "ONE" is interesting. There are two words for "ONE" in Hebrew. The first word means `a single thing by itself`, but the other word is always "A COMPOUND WORD", meaning lots of things joined together in ONE, and every time God is described as "ONE" in the Old Testament it is this COMPOUND word which is used - more than one in one. You and I may not be able to understand how there could be more than One in One, but we cannot understand how there could be more than one cupful of oil in one cup (See Study ), yet the Bible says it happened and I believe the Bible.
Also, in the Old Tesament, we see not only that God is ONE, and God TWO, bvut that He is THREE in One. The prophet Isaiah was inspired by God to put the following words into the mouth of the Messiah (Jesus) saying, "GOD has sent ME and His SPIRIT". There we see the Trinity proclaimed for anyone who has the eyes to see.
The New Testament agrees with the Old. When Jesus gives His Summary of the Commandments He quotes Deuteronomy 6 verses 4 and 5, "The Lord our GODS is ONE LORD", yet He also commands His Disciples to baptise "in the NAME" (One Name - singular) "of the Father and of the Son and of the Holy Spirit" (Matthew 28.19). When He prays, what does He pray? "Lord, I want my Disciples to be ONE, even as WE ARE ONE", and throughout prayer you have the same "US"! that describes God in Genesis!
"US" "WE", Jesus talking to His Father, praying for the same Unity for His Disciples that He has with His Father, a Unity of thought so that they may think alike, a Unity of word so that they may say the same thing, a Unity of deed (NOT that they do the identical thing, but the one complementing the other), but above all a Unity of nature in SPIRIT - that they may be One as He and God are One in Spirit.
How can we come to belief in the Trinity? By having it explained? I doubt it. The explanation of the Cruse of oil in 1 Kings 17 (see Study 3) is the nearest I have come to understanding it. Or perhaps we can say that it`s like marriage where husband and wife are ONE FLESH. The word ONE" used in this case (in Genesis 2.24) is again the compound word "ONE" - two people who are ONE FLESH. Can we add a third and begin to see that GOD IS THREE IN ONE!?