This is my attempt to explain the basic operation of the Trinity:
The Trinity is analogous to 'a ball rolling off a table.' Three segments can be distinguished. There is, the ball as it is rolling, the ball as it is falling, and the ball as it makes contact with the floor; the analogy consists of a roll, a fall, and a point of contact. God the Father is the roll, God the Son is the fall, and the Holy Ghost is the point of contact. And the kinetic energy of each segment represents each person's operation as God; they are each one unique. Filioque: Predominant and foremost is the ball as it is rolling — this is because the fall and the point of contact proceed from it; the fall and the point of contact happen within the context of the roll, hence 'a ball rolling (off a table.)' Likewise the Father is foremost, because the Son and the Holy Ghost proceed from him, they happen in context of him. (This answers the question of how God the Son could pray to God the Father, how God could pray to God.) Furthermore, from the Father proceeds the Son, and together the Holy Ghost, and thus, the Holy Ghost is preceded by the Father as well as the Son.
There is another aspect to this analogy, and that is the properties associated with each of the three segments. Of the roll there is the table itself, of the fall the atmosphere, that is resistance, and of the contact the floor; each in some way defines the segment in its operation. Likewise the Father is defined by his omnipresence, the Son by his flesh, and the Holy Ghost by his constitution of spirit.
Here are some statements of Jesus that can be made sense of using this analogy:
John 10: 39) "[...] the Father is in me and I am in the Father.”
John 14: 6–7) "[...] no one comes to the Father, but by me. If you had known me, you would have known my Father also; henceforth you know him and have seen him.”
John 16: 7) "Nevertheless I tell you the truth: it is to your advantage that I go away, for if I do not go away, the Counselor (Holy Ghost) will not come to you; but if I go, I will send him to you."
***
Paul did not invent the Trinity since it is rooted in the Old Testament; the prophet Isaiah predicts the coming of both the Son of God and the Holy Ghost, and a careful examination of his prophecies will reveal this. Take these four prophecies from the Book of Isaiah. Though they do not comprise the entirety of the prophecies concerning the persons of the Trinity they are perhaps the most straightforward and plain.
I have color coded each portion within each prophecy so that they should be examined in and of themselves. And each color coordinates a certain topic between prophecies.
PROPHECIES
Isaiah 2: 2–5) In days to come, the mountain of the LORD’s house shall be established as the highest mountain and raised above the hills. All nations shall stream toward it. Many peoples shall come and say: “Come, let us go up to the LORD’s mountain, to the house of the God of Jacob, that he may instruct us in his ways, and we may walk in his paths.” For from Zion shall go forth instruction, and the word of the LORD from Jerusalem. He shall judge between the nations, and set terms for many peoples. They shall beat their swords into plowshares and their spears into pruning hooks; one nation shall not raise the sword against another, nor shall they train for war again. House of Jacob, come, let us walk in the light of the LORD!
Isaiah 11: 1–10) But a shoot shall sprout from the stump of Jesse, and from his roots a bud shall blossom. The spirit of the LORD shall rest upon him: a spirit of wisdom and of understanding, a spirit of counsel and of strength, a spirit of knowledge and of fear of the LORD, and his delight shall be the fear of the LORD. Not by appearance shall he judge, nor by hearsay shall he decide, but he shall judge the poor with justice, and decide fairly for the land’s afflicted. He shall strike the ruthless with the rod of his mouth, and with the breath of his lips he shall slay the wicked. Justice shall be the band around his waist, and faithfulness a belt upon his hips. Then the wolf shall be a guest of the lamb, and the leopard shall lie down with the young goat; the calf and the young lion shall browse together, with a little child to guide them. The cow and the bear shall graze, together their young shall lie down; the lion shall eat hay like the ox. The baby shall play by the viper’s den, and the child lay his hand on the adder’s lair. They shall not harm or destroy on all my holy mountain; for the earth shall be filled with knowledge of the LORD, as water covers the sea. On that day, the root of Jesse, set up as a signal for the peoples — him the nations will seek out; his dwelling shall be glorious.
Isaiah 66: 1–2, 42: 1) Thus says the LORD: The heavens are my throne, the earth, my footstool. What house can you build for me? Where is the place of my rest? My hand made all these things when all of them came to be —oracle of the LORD. This is the one whom I approve: the afflicted one, crushed in spirit, who trembles at my word. 42 Here is my servant whom I uphold, my chosen one with whom I am pleased. Upon him I have put my spirit; he shall bring forth justice to the nations.
***
COLOR COMPILATIONS WITH NOTES
- In days to come, the mountain of the LORD’s house shall be established as the highest mountain and raised above the hills. All nations shall stream toward it.
- On that day, the root of Jesse, set up as a signal for the peoples — him the nations will seek out; his dwelling shall be glorious.
- Many peoples shall come and say: “Come, let us go up to the LORD’s mountain, to the house of the God of Jacob, that he may instruct us in his ways, and we may walk in his paths.” (Note the plural, "paths"; in context it clearly implies and prophesies an entourage walking from place to place and not individuals metaphorically 'walking a path (singular),' hence the phrase prior. The phrase prior conveys God issuing instruction in merely a general sense. The leader of this entourage is referred to here as "the LORD's mountain" and is the Son of God.)
- House of Jacob, come, let us walk in the light of the LORD!
- But a shoot shall sprout from the stump of Jesse, and from his roots a bud shall blossom. (Again, this passage is a prophecy about the prophet that is the leader of that entourage in Isaiah 2: 2–5. Here he is referred to as "the stump of Jesse". Further in Isaiah 11 is the prophecy that "[...] — him the nations will seek out.". And for his significance he is referred to here as a 'blossoming bud,' a term conveying beauty and/or life.)
- For from Zion shall go forth instruction, and the word of the LORD from Jerusalem. He shall judge between the nations, and set terms for many peoples. (Again with a prophecy, of nations in some way surrounding this prophet, the Son of God.)
- Not by appearance shall he judge, nor by hearsay shall he decide, but he shall judge the poor with justice, and decide fairly for the land’s afflicted. He shall strike the ruthless with the rod of his mouth, and with the breath of his lips he shall slay the wicked. Justice shall be the band around his waist, and faithfulness a belt upon his hips. (This is clearly in the context of a prophet issuing a word to the people, and who himself is in a sense THE Word.)
- They shall beat their swords into plowshares and their spears into pruning hooks; one nation shall not raise the sword against another, nor shall they train for war again. (The word of this prophet who is the"LORD's mountain" and the "stump of Jesse" and the Son of God does not at all condone state-sponsored violence under any circumstance, because the adoption of his message facilitates an absolute adherence to peace — remember that the sword begets the sword. The state embodies the principles and determination of the people.)
- Then the wolf shall be a guest of the lamb, and the leopard shall lie down with the young goat; the calf and the young lion shall browse together, with a little child to guide them. The cow and the bear shall graze, together their young shall lie down; the lion shall eat hay like the ox. The baby shall play by the viper’s den, and the child lay his hand on the adder’s lair. They shall not harm or destroy on all my holy mountain; for the earth shall be filled with knowledge of the LORD, as water covers the sea.
- The spirit of the LORD shall rest upon him: a spirit of wisdom and of understanding, a spirit of counsel and of strength, a spirit of knowledge and of fear of the LORD, and his delight shall be the fear of the LORD. (Of major significance is the mention of a spirit. The mere mention of there being a spirit lends credence to the assertion that the concept of the Trinity can be found in the Old Testament, because a person that is a spirit is a component of the triune god.)
- Thus says the LORD: The heavens are my throne, the earth, my footstool. What house can you build for me? Where is the place of my rest? My hand made all these things when all of them came to be —oracle of the LORD. This is the one whom I approve: the afflicted one, crushed in spirit, who trembles at my word. 42 Here is my servant whom I uphold, my chosen one with whom I am pleased. Upon him I have put my spirit; he shall bring forth justice to the nations. (Because this prophet is "crushed in spirit" God has said, "Upon him I have put my spirit", one of wisdom, understanding, counsel, strength, knowledge, and fear of the LORD. Also, notice the preceding passage, that it is made clear God's omnipotence and omnipresence before stating of whom he approves; the context would suggest that clarification needed to be made or else it might be forgotten, God's omnipotence and omnipresence.)
***
CONCLUSION
After a study of these prophecies peruse the prophecy of Isaiah 12. Notice that it more or less sums up the core teachings behind the whole of Isaiah's prophecies, as well as the Psalms. And notice the end of it, the phrase, "[...] great in your midst is the Holy One of Israel!" no doubt a reference to the prophesied prophet and god-man.
Isaiah 12) On that day, you will say: I give you thanks, O LORD; though you have been angry with me, your anger has abated, and you have consoled me. God indeed is my salvation; I am confident and unafraid. For the LORD is my strength and my might, and he has been my salvation. With joy you will draw water from the fountains of salvation, and you will say on that day: give thanks to the LORD, acclaim his name; among the nations make known his deeds, proclaim how exalted is his name. Sing praise to the LORD for he has done glorious things; let this be known throughout all the earth. Shout with exultation, City of Zion, for
great in your midst is the Holy One of Israel!
And here are a few other passages to go with the phrase, "[...] great in your midst is the Holy One of Israel!"
Isaiah 65: 1) I was ready to be sought by those who did not ask for me; I was ready to be found by those who did not seek me.
I said, “Here am I, here am I,” to a nation that did not call on my name."
Isaiah 52: 6) "Therefore my people shall know my name; therefore in that day they shall know that
it is I who speak; here am I.”
Isaiah 58: 9) "Then you shall call, and the Lord will answer; you shall cry, and
he will say, Here I am."
FINAL REMARKS
Isaiah no doubt prophesies the coming of a man who will claim to be God, that is for certain. What remains to be the question is whether you believe Isaiah to be a true prophet? The Quran makes no mention of him, and perhaps this is why, because these prophecies are Islamically not deemed valid.