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? Reason alone will never give us answers regarding the nature of God. If such answers are available, they will have to come from the self-revelation of God. What does the Bible say about the nature of God?
"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, the great Medieval Jewish scholar, 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 Genesis 1:26 and 3:22, God refers to Himself in the plural ("Let us ..."). Jewish scholars dismiss these hints of plurality, but they are nevertheless present in the ancient text.
Judaism, Islam and Christianity are all monotheistic systems. However, only Christianity's trinitarian theology has a defense against the tendency toward monism.