Who is "Akum" עכו''ם עובד כוכבים ומזלות [idolater]? This is an argument among rishonim. The Rambam (as is well known) holds Christians are in that category. To Tosphot in Tractate Avida Zara, they are not because "shituf" ["joining" with God ] is not idolatry. This comes up because this argument is often ignored and people think the Rambam is the only opinion. That is just not so.
However the particular Tosphot that deals with this issue seems to have about three separate reason for the fact that Christians are not idolaters. [But besides his depending on that Gemara itself that Shituf is not idolatry, I forgot the other reasons.]
I might add here that Aquinas spent a good deal of effort in defining and defending the idea of Divine simplicity. He shows that according to the Torah, God is "simple" which means not a composite. And he shows that that does work within his context. Divine simplicity is central to Torah faith as no one in the Middle Ages disagreed with.
[Divine simplicity was a major issue in the middle ages and in fact probably was the reason that the Platonic model of Emanation was dropped and the Aristotelean model was adopted.]
