There issue of Christianity I thought was in Avoda Zara but today I was at the near by Na Nach Breslov place and saw a book by the son of Rav Odaviah Yoseph [about using medicine on Shabat]]and hebrought up the fact that Muslims are not idol worshipers and the mentioned that famous Tosphot about Christianity and "Shituf" {joining}.  He brings in from Sanhedrin page 63 and the Rema {Moshe Iserles} in the Shulchan Aruch in Orach Haim chapter 156.

{I did not look these up but I guess I was wrong about the location of that Tosphot.}