According to Islamic theology, the belief in a single God (as opposed to polytheism or idolatry) is a natural disposition of the human mind, a concept referred to as Fitrah. Did a similar idea existed in the pre-islamic Christian world?
Paul clearly expresses, Romans 1:18-26, the concept of humanity, by origin and by birth, having an innate instinct that God is and that there is but one God.
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; 19 Because that which may be known of God is manifest in them; for God hath shewed it unto them. 20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
Paul also clearly indicates that humanity, as a whole, has not responded to that innate instinct found within them.
21Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened. 22Professing themselves to be wise, they became fools, 23And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
And Paul shows, therefore, that humanity is without excuse if they do not worship Him of whom they are, by nature, aware. And God is righteous to give up the humanity he made because of their behaviour.
24Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: 25Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
Romans 1:18-26 KJV
The Christian Bible states this fundamental belief that humanity knows there is a God in Romans 1:18-26, despite the fact of idolatry in every age, including the time when the apostle Paul made that statement. His point is that humanity is without excuse if they deny the existence of God, or if they turn their worship of created things into idolatry. Idolatry and polytheism still depend on belief in the existence of deity, being but a corruption of that basic fact.
Further, Christianity grew out of Judaism, which also believes that all humanity knows there is one God, even if some deny that. The Judaic Hebrew scriptures are full of this belief that there is one God.
Christianity agreed, and although the Christian Greek scriptures expanded on the complexity of the one Being of God, belief in one God is just as essential to Christianity as it is to Judaism. When the Islamic religion was formed at the start of the 7th century A.D., Muslims also knew that belief in one God was essential. All three world faiths view atheism as an aberration (dictionary definition - 'a straying from the path').
So, in order of appearing on the world stage, Judaism took it as understood that all humanity knew there was a God (even if many believed in multiple deities), then Christianity emerged some 2,000 years later and continued to think of humanity knowing this one God exists, with Islam appearing some 500 years later and also taking it as understood that humanity knew one God existed.
One early Christian writer, Tertullian (circa 160 to 230 A.D.) has left his written explanations of the one Christian God, in Against Praxeas 2. There, he explained the Christian view of organic monotheism. Disbelief in the existence of God is so rarely mentioned (up until more modern times) that it is safe to say that billions of humans, for over 6,000 years, have had a natural inclination to believe that there is one God, or, with pagans, many of them (which still requires belief in God). I mention Tertullian as I suppose this fits in with your question's mention of 'late antique Christianity'