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!?
Yes, you CAN come to belief in the Trinity by having it explained, and seeing that the Trinity is there in the word of God, but you can also come the way the Apostles came, the way that will make it YOUR faith as it became THEIR faith, and that is by the way of EXPERIENCE. You may have believed in your mind that God is ONE through the teaching you have been given down many years, but that will not necessarily excite and inspire you. One day, however, by the grace of God you will realise that Jesus came to save you - that He came searching for you, the lost sheep - that He died for YOU on the Cross to save you for eternal life. That is the GRACE OF OUR LORD JESUS CHRIST, and when you really see it for the first time, you begin to realise that a MAN could not have done that for you, only GOD could have done that - and you`ve found THE LOVE OF GOD. Then you begin to discover that you are not the only one that this has happened to, and that there are countless other people who have found the grace of Christ and the love of God, so that you are "born again" and "in a family", and as you meet with them you find THE FELLOWSHIP OF THE HOLY SPIRIT.