The Trinity is a Christian doctrine, stating that God is one Being Who exists, simultaneously and eternally, as a mutual indwelling of three persons (not to be confused by "person"): the Father, the Son (incarnate as Jesus of Nazareth), and the Holy Spirit. Since the 4th century, in both Eastern Christianity and Western Christianity, this doctrine has been stated as " three persons in one God," all three of whom, as distinct and co-eternal persons, are of one indivisible Divine essence, a simple being. The doctrine also teaches that the Son Himself has two distinct natures, one fully divine and the other fully human, united in a hypostatic union. Support of the doctrine of the Trinity is known as Trinitarianism. Most denominations within Christianity are Trinitarian, and regard belief in the Trinity as a mark of Christian orthodoxy.