The Trinity Study Series. The Christian doctrine of the Trinity teaches the unity of Father, Son, and Holy Spirit as three persons in one Godhead. The doctrine states that God is the Triune God, existing as three persons, or in the Greek hypostases, but one being. Each of the persons is understood as having the one identical essence or nature, not merely similar natures. The doctrine of the Trinity has been stated as "the one God exists in three Persons and one substance, Father, Son, and Holy Spirit."