If God of the Old Testament is the creator and lord of the universe - "In whose hand is every living soul and the breath of all humankind", then why is a mediator i.e. Jesus required? Isn't that just making things more complicated? In other words, when you turn to God, who are you turning to?
And if you want to say that the trinity is an inherent feature of God and therefore Jesus makes sense in that context, then why do you need a physical representation of one part of that trinity? If you already have notion of God then what's stopping you from developing a notion of a non corporeal component of the third part of that trinity?
Its a profound question and obviously from a Christian perspective the answer is going to be: Yes.
But the very brief summary of why is: God had a plan, part of the plan involved several different covenants he established with his people.
From a very high level view, those covenants each proceeded to include a larger and larger group. It started with Adam and the covenant was between God and one man, then it moved to Noah (a family), then Abraham (a family + all descendants), Moses (a people), David (a kingdom), etc.
The final covenant was offered to all mankind and this covenant was brokered by "his son" (a strange thing to say about a trinity, but our words lack the power to describe it any better)....the importance of the sacrifice of his son goes back to the original opening of the Bible.
God instructs Abraham to sacrifice his son Issac, which he agrees to (but God stops him and instead tells him to sacrifice a nearby lamb). The final covenant then bookends this offer, with God himself accepting to come as a human, suffer the trials of humanity, and have humanity abuse, torture, and kill him as a provable sign of Gods love for us.
So if you accept that: God has a plan that was meant to go A -> G and if you ask "Must I accept G"...well yes you kinda do. Furthermore, step G was the culmination of the entire Bible. Further if you accept that God loves you so much that as proof he offered to come be tortured on this earth so that you might have a provable role model to follow, then yes you must accept Jesus under this framework.
The Hebrew Bible makes it clear innumerable times whether explicitly or implicitly that the covenant between God and Israel has relevance for the gentiles and is actually meant to provide the "nations of the world" access to God. "ונברכו בך כל משפחות האדמה" was literally the first message communicated to Abraham. I can bring you similar examples from practically every book of the Pentateuch, prophets and writings if you so desire.
So the concept of an additional covenant revealed hundreds of years after the last prophets were active to deliver such a message is very questionable. Never mind that this covenant was composed in Greek a second or third language at the time for Jews many years after Jesus' life and death. Also, Jesus presented himself as a messiah for the Jews first and foremost. The concept of salvation for gentiles was a later development of Paul and the status of Jesus as the son of God was an even later development. My point is that an indeterminate monotheism without a flesh and blood mediator would be more relevant for the modern Christian. Such a mediator has no basis in the covenants described in the Hebrew Bible and also is more compatible with science and logic.
I'm just trying to understand why Christians insist on Jesus as a mediator when he's clearly not necessary...
Are you Jewish? Your belief system certainly is.
By 30AD, God stopped accepting Jewish sacrifices. And since then, Jews have been forced to roam the earth as a diaspora and live as resident aliens in foreign nations.
This is not Christian interpretation, as it comes direct from the Talmudic testimony. It states that for 40 years before the destruction of the Temple (the destruction of the Temple happened in 70AD, so 40 years before exactly corresponds the crucifixion of Christ):
The animal sacrifices were no longer accepted, the thread of the scapegoat would no longer turn white, the outermost lamp would not light the temple, and its doors would not close. This then culminated with the actual physical destruction of the temple in 70AD. (its also quite telling that "the doors of the Hekhal" would not close, a clear symbol that the covenant of Christ, expanding God to all people was no longer controlled by ethnic Jews nor its priests).
It would seem that a sincere student of Judaism would recognize this situation, but its not permanent: You can accept Christ and be welcomed back by God. Otherwise you can "pick and choose" what parts of the Bible you want to read and remain a self-imposed orphan.
I think part of the difficulty is thinking of God in separate parts. A being that can be three and one at the same time isn't bound by the physics of physical being. Man's rules of nature don't apply.
Not in the physical world. And Jesus was a man of flesh and blood. To say that he was God is simply idolatry