Very early in life Newton abandoned orthodox belief in the Trinity. At this time the Socinians were an important Arian sect amongst intellectual circles. It may be that Newton fell under Socinian influences, but I think not. He was rather a Judaic monotheist of the school of Maimonides. He arrived at this conclusion, not on so-to-speak rational or skeptical grounds, but entirely on the interpretation of ancient authority. He was persuaded that the revealed documents give no support to the Trinitarian doctrines which were due to late falsifications. The revealed God was one God.
J. M. Keynes, “Newton the Man” in The Royal Society Newton Tercentenary Celebrations 15-19 July 1946 (Cambridge, 1947), 27-34