Holy Trinity, Batman!

The Catholic Encyclopedia's entry on "The Blessed Trinity", which if I remember correctly is quoted in The God Delusion, explains the whole concept as clear as mud. In fact the other religions might consider it something of a feather in their cap that they've avoided this particular muddle:

The Trinity is the term employed to signify the central doctrine of the Christian religion — the truth that in the unity of the Godhead there are Three Persons, the Father, the Son, and the Holy Spirit, these Three Persons being truly distinct one from another.

Thus, in the words of the Athanasian Creed: "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God." In this Trinity of Persons the Son is begotten of the Father by an eternal generation, and the Holy Spirit proceeds by an eternal procession from the Father and the Son. Yet, notwithstanding this difference as to origin, the Persons are co-eternal and co-equal: all alike are uncreated and omnipotent. This, the Church teaches, is the revelation regarding God's nature which Jesus Christ, the Son of God, came upon earth to deliver to the world: and which she proposes to man as the foundation of her whole dogmatic system.

(...)

The first creed in which [the word] appears is that of Origen's pupil, Gregory Thaumaturgus. In his Ekthesis tes pisteos composed between 260 and 270, he writes:

There is therefore nothing created, nothing subject to another in the Trinity: nor is there anything that has been added as though it once had not existed, but had entered afterwards: therefore the Father has never been without the Son, nor the Son without the Spirit: and this same Trinity is immutable and unalterable forever (P.G., X, 986).

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