The issue of Christianity comes up in a few places. Saadia Gaon, Rav Avraham Abulafia, Rav Yaakov Emden, the Meiri, and a Tosphot in tractate Avoda Zara, and the Ari. [That Tosphot is fairly well known because it is the source of the idea that gentiles are not commanded on the prohibition of "shituf". ["joining"]. However I forgot the page number. But it should be easy to find for anyone who is interested. You just look up the Rema in the Shulchan Aruch of Rav Joseph Karo in laws of oaths and the Beer HaGola will bring the page number of that Tosphot.]
Rav Saadia Gaon brings two problems. That of identifying God and Jesus. Plus nullification of the commandments. Both of these points are correct.
Rav Avraham Abulafia has a very positive opinion about Jesus, but a highly negative opinion about the Catholic Church. That is more of less the same as Rav Yaakov Emden. The Ari has the same opinion as Rav Abulafia as you can see in the writings of the Ari on the end of Genesis, concerning the burial of Joseph in Egypt. [That is the idea of the Ari is identical to that of Rav Avraham Abulafia.]
[My own opinion is that I have no opinion except what these sages say. However there are some other opinions that disagree with the above, and when there is a difference of opinion among the rishonim I say אלו ואלו דברי אלקים חיים אבל הלכה כהרוב These and those are the words of the Living God, but the law goes by these that I have quoted since they are the majority.]
