When I read  the introduction of Maimonides to the Mishna, I was surprised to see that he had already at the beginning of his life laid out his plan about what he was going to write. He already had the basic outline of the Yad HaChazaka (Mishneh Torah) and the Guide for the Perplexed already laid out in his mind.
This reinforced what I was anyway thinking about the Rambam that his switch to Aristotle was intentional and meant to clarify the issue of idolatry. He meant it from the beginning, and it was not some fluke at the end of his life.

He wanted the difference between idolatry and Monotheism to be sharp and distinct and not dependent on degrees. Of course you could ask who does not want that? Everyone wants that! But my point is no one could get it. With Nachmanides or any Neoplatonic system, it is completely arbitrary where you draw the line between godliness and not godliness. Obviously the Rambam meant right from the beginning to stamp that out.
As long as you believe in Emanation, then anything you want can be godliness. And you can conveniently say the line stops where you want it to stop so that your system is conveniently kosher and everyone else's is not.  Perfect. [I don't claim that was the only reason the Rambam switched to Aristotle. The Neo Platonic systems had anyway been tried and failed. I don't recall what the problems were off hand. Maybe the third person problem was one thing.(Which is only a problem if you consider substance to be not a composite.)]

At any rate, we do have Nachmanides with his Neoplatonic approach, which does tend to balance the playing field.

And this leads to the question about שיתןף "joining" in Tosphot Sanhedrin 63b.

 I am not sure what that Tosphot means. He says one can have a business with Christians because of 1) when  they swear by their holy things, they don't intend godliness, 2) when they mention Jesus, they intend the Maker of heaven, 3) they are not commanded about "joining."
So far I have not been able to make heads or tails about what Tosphot means here.

I am guessing that maybe in the Middle Ages people would swear by the wafer. The second thing seems to be dealing with the Trinity of Athenius.  The third thing seems to be some kind of idea about Emanation because otherwise why would they not be commanded not to do joining?

And it is hard to know why Christianity would be "joining" שיתוף. Joining we know from page 63a means to join something else to God. What Christians do is say x=y. That is not the same as x+y.

1] I should mention just to clarify that saying someone is the son of God is not a problem because the Torah does this all the time.  בנים אתם להשם אלהיכם, שלח את בני ויעבדני, בני בכורי ישראל "You are the children of the Lord your God," "Send out my son so that he will worship me." "Israel is the first born of God." "My son, my first, born Israel." So if all the Jewish people are children of God, then specifying one particular member of the entire set as a son of God is not an exception to the rule.
So if your father says, "These three boys are my children" and then says "This boy [who is in the set of all three boys] is my son," there is no contradiction because he was already part of the entire set.
2] Tosphot is not referring to Roman Catholic doctrine after Aquinas. Rather to pre-Aquinas doctrine which was Neo-Platonic.
3] We can see why pantheism would be a problem. Not only is it not what the Torah is saying, but also it has this aspect that the Rambam did not like about  Emanation. [To the Rambam there is no emanation.]
4] To the Rambam God is the First Cause. He is not a composite.
5] It is not just that I do not understand the individual points of Tosphot. It is also I don't know how he is combining his points.
6] We find for example that the Ari considers the souls of people like the patriarchs to be from the world of Emanation. And that we know from the Zohar and the Ari is Godliness. To the Ari the bottom of Emanation is where godliness ends and creation begins. That is explicit in the Zohar. And in the Shaar HaGilgulim of the Ari and Reb Chaim Vital we find this theme extended greatly. We find even Bava Sali said about his son, Meir Abuchatzeira that his soul was from Emanation. So it is not unusual to claim someone is Divine. No one considered it even to be a theological problem. Mainly because people form their ideas based on group identity, and not because they actually think about the implications of their beliefs.
7] So the venues of future exploration are the Gemara in Suka and the other Tosphots that bring up this issue.   The Gemara in Suka asks on the Mishna the question of "joining."
8] My own opinion I should mention is like that of the Rambam. God is one, not two and not three, and not a composite. And I don't think anyone's soul is Godliness. But I am willing to accept that some people are divinely inspired like Moses and the prophets.