Why was Newton a theist?

[Anomalocaris]

Isaac Newton, who practically invented physics and calculus, is easily one of the most intelligent people to have ever lived. But he was also a theologian and spent decades studying the bible, theology and church history. Why didn't he use his rationality to denounce Christianity and become an atheist? He lived before Darwin; is this sufficient to excuse his belief in the Christian god?

The received wisdom of the age said the gap between what can plausibly occur without intelligent design and what actually exists is so vast that intelligent design seem easily the less presumptious assumption.   It took another 150 years before careful study of what actually transpires in nature to illuminate the gap between what can be demonstrated and what is need for naturalistic origin of everything is much narrower and less clear cut then it previously seemed, while the implicit presumption of intelligent creator with no demonstrable backstory creation became ever more overreaching by comparison.