Trinity, central doctrine of Christian theology, that there is one God who exists in three Persons and one Substance. The definition of the doctrine, implicit in the New Testament, by the early ecumenical councils (notably Nicaea and Constantinople) was the product of violent controversy with heresies like Arianism, Monophysitism, Nestorianism, and Monarchianism.It is classically summed up in the Athanasian Creed. The three Persons—the Father, the Son (incarnated as Jesus Christ), and the Holy Spirit—are each fully God: coequal, coeternal, and con substantial, yet distinct. The Son is “eternally begotten” by the Father; the Holy Spirit “proceeds” from the Father and (in Western theology) from the Son. The doctrine is a mystery, being known by revelation and being above reason (though not unreasonable). Hence it has been challenged by rationalists and by sects like the Jehovah's Witnesses and the Mormons.