IBN SIDA
Ibn Sida (980-1037) In Ibn Sida's day, among Jews and Arabs, there were thinkers known as 'Faylasufs' - Philosophers.
Like the Faylasufs, Ibn Sida never had the slightest doubt that God existed.
Ibn Sina spoke of God in terms of the necessity of being (wujub al-wujud), and 'necessity' for Ibn Sina was a so-called 'primary intelligible', a concept so fundamental as to lie beyond demonstration.
The Qur'an seems to confirm this view: "And this was revealed to Noah: Never will any of your people believe, except those who have already believed.'
The Qur'an states strongly that: Those who reject theism through lack of understanding, have been kept from it by God, and those who accept it through understanding, have been given it by God.
Ibn Sida worked out a rational demonstration of the existence of God based on Aristotle's proofs which had become standard in Judaism and Islam.

Aristotle (384-322) went further even than Plato(427-346 BCE), in his belief that the Universe was essentially rational.
Aristotle was the first to appreciate the importance of logical reasoning, the basis of science.
The Unmoved Mover
Aristotle developed a hierarchy of existences, each one of which imparts form and change to the one beneath it
But, unlike in the old myths, the emanations grew weaker and weaker the further each entity was from their source.
At the top of the hierarchy was the Unmoved Mover.
The Unmoved Mover causes all motion and activity in the universe, and activates a process of attraction - all beings are drawn towards Being itself.
The human soul has the divine gift of intellect, the possession of which makes Man akin to God, the Unmoved Mover, immobile and spritual, simultaneously thinker and pure thought
Matter is flawed and mortal, there is no material element in Aristotle's God. God is outside matter and temporal activity.
God did not create the world! Aristotle's God was not, strictly speaking, relevant to religion...

Ibn Sida begins with a consideration of the way our minds work. We categorise, analysing things by breaking them down into their component parts, dividing things up until no further division is possible. (Or at least if we are in thrall to Aristotle we do)
Everything is dependent, contingent on everything else.
Except that which is Simplicty Itself. That Uncaused Being, the Unmoved Mover is at the apex of the hierarchy of existence.
Either the chain contains only that which is contingent, or it also contains that which is non-contingent. If the chain only contains contingents, it might be finite or infinite.
But it will require an external cause in either case.
Ibn Sina himself was of course notorious in Islamic intellectual history, for believing that time and matter were incapable of beginning.
Nevertheless he held that a beginningless cosmos, consisting in nothing but contingent states, cannot dispense with an explanation just because it is beginningless.

The contemporary physicist Stephen Hawking presents the same point in discussing the idea of a 'no boundary condition', in which space-time is open, and not limited by singularities. He says: "So long as the universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it
would simply be. What place then for a creator?"
It seems fairly clear that Ibn Sina's argument in its essentials, corresponds with the Leibniz-Spinoza cosmological argument, and prefigures it by seven centuries. For it too seems to argue that infinity is irrelevant in the face of the fundamental imperative that what is contingent in itself presupposes what is necessary.
Ibn Sina's proof is something of a philosophical breakthrough.

EXTRACT FROM NOVEL: GRAND STREET
By Ann Magazine Goldstein and Giorgio Manganelli
Around ten in the morning, a man of serious learning and somewhat melancholy humor had discovered the irrefutable proof of the existence of God. It was a complex proof, but not so complex that it could not be grasped by a moderately philosophical mind. The learned man remained calm, reexamined the proof of the existence of God from end to beginning, sideways, and from beginning to end, and concluded that he had done a good job. He closed the notebook containing the notes relating to the definitive proof of the existence of God, and went out to occupy himself with nothing--in short, to live. Around four in the afternoon, returning home, he realized that he had forgotten the exact formulation of some sections of the demonstration; and every section was, naturally, essential.
The situation made him anxious. He went into a bar to have a beer, and for a moment seemed to himself calmer. He recovered one section, but immediately afterward realized he had lost two others. He placed his hope in his notes, yet he knew that the notes were incomplete, and he had left them that way, since he did not want anyone, even the maid, to be certain of the existence of God before he had carefully worked out the full argument. Two-thirds of the way home, he realized that, as the proof of the existence of God was losing its solid, miraculous characteristics, he was running into arguments that he was no longer certain belonged to the original argument. ... Arriving at the door of his house, he was bathed in a cold sweat. Of what, really, had he demonstrated the existence? For he had reached a result that was indisputably true, invincible, and yet impossible to fix in an unforgettable formula.

Frank Tipler, a physicist who earned his PhD on the physics of time travel, wrote down his own idea of what God might be in his book "The Physics of Immortality".
Assuming that the Universe will some day stop expanding and recollapse into a what is called a Big Crunch, he argues that beings near the end of time will have the data processing power to reconstruct everybody who ever lived, and give them a second life which subjectively will last forever: a virtual reality afterlife.
Tipler's God is a computer, and He is not dead - in fact, he has yet to come into existence.