All Jews, Muslims, and Christians affirm that God is one. But this statement needs more clarification. Unity is not the same as identity. Is God a single entity, or is there some sense of diversity also in God? We need to examine the Bible for the answer.
"Hear O Israel, the Lord your God, the Lord is one." (Deut. 6:4). This verse, the Shema, clearly states the oneness of God. However, even here there is an intimation of the Trinity. Moses Maimonides noted that the name of the Lord is repeated three times. Also, the word for one is achad, which means a "unified one", as distinct from rashid, which means a "single one".
In Genesis 1:1, the name of God is Elohim (a plural form of El ). In 1:26 and 3:22, God refers to Himself in the plural. Jewish scholars dismiss these hints of plurality. Judaism, Islam and Christianity are all monotheistic systems. However, only Christianity's theology has a defense against the tendency toward Monism. See the Overviews for more discussion of this.