There is no doubt whatsoever that Newton and others (including Copernicus) were sincere in their theism and not just using the ubiquitous “God language” of the day. Any discussion as to whether Newton was a true Christian (by someone’s standards) is a red herring. Newton struggled with the Trinity–I am not actually aware of has flat out denial–but in any event it is irrelevant. He was a theist. Not to mention that many modern Christians who affirm trinitarianism struggle with the Trinity. If they (we) think about it deeply, we should be forced to admit it is an impenetrable mystery. I am not sure that Newton was not in a similar place, but being Newton he examined it more thoroughly, and perhaps this comes across and being anti-Trinitarian. But the bottom line is that it is irrelevant.
Athanasian Creed
Apart from the opening and closing sentences, this creed consists of two parts, the first setting forth the orthodox doctrine of the trinity, and the second dealing chiefly with the incarnation and the two-natures doctrine.