I wish I could remember the page in Tractate Avoda Zara where the issue of "joining" comes up. I just recall that the issue had to do with the fact that gentiles are not commanded concerning joining [i.e  joining the name of Heaven with another.] An example is brought in the Gemara there of Gideon "A sword to God and to Gideon". There you see Gideon himself joining the name of Heaven with his own name This the Gemara concludes is forbidden to a Israeli but allowed for gentiles.

Why is that page important? Because of the Tosphot there that brings up the issue of Christianity   and I thought it is such an important Tophot that I asked my learning partner to skip what we were doing at the time and instead concentrate on that Tosphot.

From what I recall Tosphot seems to have three different approaches there.