What religion was sir Issac Newton?

Answer 1

He was an unorthodox christian.

Newton wrote a number of religious tracts dealing with the literal interpretation of The Bible, as he considered himself to be one of a select group of individuals who were specially chosen by God for the task of understanding Biblical scripture.[3] Newton's conception of the physical world provided a stable model of the natural world that would reinforce stability and harmony in the civic world. The law of gravity became Newton's best-known discovery, but Newton saw a monotheistic God as the masterful creator whose existence could not be denied in the face of the grandeur of all creation.[4][5]

Although born into an Anglican family, by his thirties Newton held a Christian faith that, had it been made public, would not have been considered orthodox by mainstream Christianity[6]; in recent times he has been described as heretical to orthodoxy

-Wikipedia