The problem regarding the ‘nature’ and identity of God is not so much one that lies at God’s ‘feet’, but one that lies at the feet of human comprehension and agenda.
It is not God that differs, but the human perception of what is the Divine that differs.
The language of the Old Testament/Torah is repleat with clues/references to personas within God’s divine singularity, and yet even the Jews themselves appear to have overlooked/misunderstood.
The word Elohim is used thousands of times for “God”; Adonai is used hundreds of times for “Lord”; both of these words are plural nouns in Hebrew.
A number of passages speak of the “faces” or “presences” or “persons” of God (Exodus 33:14; Deuteronomy 4:37; and Job 13:8).
God refers to Himself as “Us,” “Our,” and “We” (Genesis 1:26, 2:18 (LXX), 3:22, 11:7; Isaiah 6:8, and 41:21-24),2 a phenomenon that is reflected in virtually every English translation.
The OT says of God, “they caused me to wander” (Genesis 20:13), “they appeared” (Genesis 35:7), “they drew nigh” (Deuteronomy 4:7), “they went” (2 Samuel 7:23), and “they judge” (Psalm 58:11).
The OT calls God our “Creators” (Ecclesiastes 12:1), “Makers” and “Husbands” (Job 35:10; Psalm 149:2; Isaiah 54:5).
The OT says that God is “holy” (Joshua 24:19; Proverbs 9:10, 30:33), another plural.
If the prophetic authors of the Bible were unitarians then surely they wouldn’t have referred to God in this way. Indeed, unitarians do not typically speak this way in ordinary conversation and fall all over themselves trying to explain them when they are brought up.
Perhaps it is a case of Jeremiah 5:21 - Hear now this, O foolish people, and without understanding; which have eyes, and see not; which have ears, and hear not:
Or, Mark 8:18 - You have eyes! can you not see? You have ears! can you not hear? and have you no memory?
So what chance does early Islam have, out on the very fringes of monotheism, on the fringes of Judaism, Christianity, Zoroastrianism, and the ‘Hanif’ {early monotheist Arabs according to Islam - but used earlier by Jews and Christians in reference to ‘pagans’ and applied to followers of an old Hellenized Syro-Arabian religion and used to taunt early Muslims.], that the Muslims believe/fantasise has a direct link between Abraham and Muhammad - this despite Muhammad and Islam clearly still clinging on to many of the old [polytheist] trappings as well. No wonder those who deeply study and ponder the issue of the ‘advent’ of Allah often also percieve this ‘god’ to be a splice-up and ‘cut and paste’ variant with an Arab-centric core.
So is the Muslim ‘Allah’ really the same as the God spoken of by Jesus to the Samaritan woman in John 4:21-23, “You Samaritans worship what you do not know; we worship what we do know, for salvation is from the Jews … the true worshipers will worship the Father in spirit and in truth for they are the kind of worshipers the Father seeks.”?