I’m going to try to avoid writing a book, but still say why I think the Trinity makes sense to someone committed to the shema.
1) Christians have generally felt the Jesus shows us God, and that God was in Christ, including in his death. That gives us a God that isn’t just the unmoved mover, but had something like the obedient Son within his experience.
2) 1st Cent Judaism also felt a need to understand God in a way that included that kind of complexity. The Word in John 1 is based on a whole Jewish tradition, e.g. Philo, but also 1st Cent Jewish understanding of the figure of Wisdom in Proverbs. This thought resulted in various intermediaries between God and us. Some were not divine, but exalted men. Others were sort of God but were spoken of as distinct, e.g. Wisdom. Both approaches occur in the NT. I'm convinced by Kirk that the Synoptics see Jesus as an exalted man.** But John and Paul see him as a human embodiment of Wisdom, which is God in his immanent form.
3) Christian theology in the West has generally given priority to God being one. Augustine saw him as one, but with enough distinction to allow for the relationship of love. The three persons were in effect the two ends of the relationship, with the Holy Spirit being sort of the presence of the Father with the Son. The pre-Vatican 2 Catholic Encyclopedia speaks of the Trinity as “the same mind will have a three-fold consciousness, knowing itself in three ways in accordance with its three modes of existence.”
4) The most sensible modern model for God seems to be panentheism. This says that God is both outside the world and within it. This would seem to result in God existing in two different ways.
5) Nicea leaves a lot of flexibility. It says that the Son is of the same ousia as the Father. This can be, and was, understood in various ways, including two beings of the same kind, and a single being.
6) Scripture speaks of Christ in a way that matches the 1st Cent Jewish speculation. John 1 uses the Logos. Several passages refer to Christ as preexisting. Assuming they understood that his actual human body was born in the 1st Cent, this seems to mean something like John 1, as saying that the human was a form of something that always existed, and was part of God.
So I think it is reasonable to see God not as a pure monad but having within his experience two (three?) kinds of existence, one the transcendent creator, but one enough like us that the human form of it can form a model for us.
Note that I’m not arguing that three persons with one essence is that ideal way to describe this. But given the concepts available in the 4th Cent, it's probably the least bad way. No one claims that we can completely understand God, so we'll always have an approximation, given our own conceptual tools. So it's no insult to say that I don't find neo-platonic language useful. Nevertheless, I try to respond to the same concerns that led to it, and not to produce a modern version of a classic heresy.
(Note that the Trinity really developed to talk about the relationship between God and Christ. It always accommodated the Holy Spirit, but that was never the focus of discussion. The actual creed from Nicea has a paragraph on the Father, a paragraph on the Son, and then “and in the Holy Spirit.” Period. The version we use in church is a later expansion.)
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** J.R. Daniel Kirk, "A man attested to God." Referring to this book can get one banned from CF. But I think we have to be honest enough to admit that the Biblical writers often show several ways of thinking about the same thing. I think there's good reason to follow John and Paul here, but it's not clear that the Synoptics do so.
