The doctrine of the Trinity has long been taught in both Catholic and Protestant churches as the central doctrine of Christendom’s religions.
... According to “The New Schaff-Herzog Encyclopedia of Religious Knowledge,” pagan Greek philosophers influenced Christendom’s teaching: “The doctrines of the Logos and the Trinity received their shape from Greek Fathers, who, if not trained in the schools, were much influenced, directly or indirectly, by the [pagan] Platonic philosophy.” (Vol. 9, p. 91)