Christianity inherits monotheism from the Jewish faith, as Christianity primarily is – Messianic Judaism. Therefore, before we begin our quest in solving the title’s question, we must ask another important question. Is Judaism monotheistic? The answer for our purposes is yes. Both Muslims and Jews can agree on the Shema Yisrael which is found in Deuteronomy 6:4, it reads:
Sh’ma Yis’ra’eil Adonai Eloheinu Adonai echad.
Hear, Israel, the Lord is our God, the Lord is One.
This is essentially the same as what is presented to us in Surat al Ikhlas (Ch. 112) in the Qur’aan:
Say: He is Allaah the One.
If we are to be honest and if we wish to examine Christianity as the post-Christ Judaic faith of which it presents itself as, then our understanding of monotheism must be based on the Shema Yisrael. The problem therefore is, does Christianity adhere to the Shema Yisrael?