The prayer Brich Shmeh from the Zohar Vayakhel 206a, which is recited during the Torah service, contains the statement ולא על בר אלהין סמיכנא. All the translations I've seen render "bar elahin" as "angel" following Daniel 3:25, but since it literally means "a son of God" I can't help but suspect that it is a reference to Christianity. Is that what's going on here, and if so, why do the translations always leave it as "angel"?
While I have seen online claims that it refers to Christianity (such as here) the connection to it as a posuk from Daniel and a similar connection to tehillim, and even the linguistic statement that "bar" means "outside of" or "except for" (as spoken of here) are as compelling, as is the historical fact that the idea of a leader (religious or secular) being the "son of god" predates Christianity's godhead including Alexander the Great.
Is the expression “we do not rely on bar elahin” a reference to Christianity?
No. It is a reference to the ancient Jewish tradition of Moses ascending to Heaven1 to take the Torah down to earth, from the Angels (because that's what Vayakhel is about), which are called sons of God in several scriptural passages (such as Job 1:6, 2:1, 38:7; see also the Jewish Encyclopedia article on angelology), so that men would not henceforth depend upon the ruling of the heavenly court in matters of Law (see not in Heaven), but rather on the earthly beth din.
1 The ancients, whether Jewish or pagan, saw mountains (Sinai, Olympus, Atlas, etc.) as stairways to heaven.
Why do the translations always leave it as angel ?
Because, in Judaism (just as in Christianity, for that matter), there are many types or groups or classes of angels (cherubim, seraphim, ophanim or galgalim, etc.), and the bene elohim form only a small part of a much larger picture (once again, please consult the aforementioned Jewish Encyclopedia article); in other words, such a translation would probably have been considered too technical or obscure, whereas angel(s), on the other hand, is (for most people, at least) much simpler, clearer, and more straightforward. (Hope this helps).