What the Christian Trinity means is fairly evident to anyone—anyone except Christians, that is. It is the divine family: a family comprising the Father, the Son and, plainly, the Mother. The all-male clergy mustn't have liked the idea of worshipping a goddess, and indeed now the very word sounds pagan; so about the time when they were busy making up the Nicene creed and kicking Arius and other heretics out of the Church, they managed to seemingly remove the Mother-of-God from the divine Trinity and replace Her with a placeholder, theHoly Ghost. Nobody knows what that Holy Spirit is supposed to be: obviously, as a hypostase, it was just fabricated from a few vague references in the Gospels where the phrase is used as a propitiatory saying when baptising (the blasphemy against the Holy Ghost shall not be forgiven unto men). The real third member of the Trinity, in fact, the most important member of the Trinity, is the Mother. No religion is originally without a female deity. Already the Jews had been deft in suppressing or disguising references to the mother-goddess Asherah, consort of El, in the Torah, though Genesis 1:27 still says that Elohim (אֱלוֹהִים—grammatically plural) created man and woman in their image. Some Protestant branches adore only Jesus and were thus quite successful in extinguishing mariolatry; the Eastern Orthodox churches followed a similar route. But the Catholics? For them, the removal was only seemingly achieved. For they don't pray to the Father—they don't even so much pray to the Son: they pray to the Mother-of-God, and visiting any staunchly Catholic country makes it completely obvious who the third and highest part of the Trinity is: just count the number of Santa Maria this and that. But those are only the dominant sects. Many more must have worshipped the Trinity as it originally stood. In fact, the point is made black on white in the Qur'an (sura 5 verse 116,يٰعِيسَى ٱبْنَ مَرْيَمَءَ أَنْتَ قُلْتَ لِلنَّاسِ ٱتَّخِذُونِي وَأُمِّيَ إِلٰهَيْنِ مِنْ دُونِ ٱللّٰهِ): Christians are accused of being tritheistic because the have put two other Gods beside Allah—Jesus and his Mother. The Qur'an may be mistaken about many things, but on this count it is clearly right.
Je reconnais avoir totalement plagié, en écrivant ça, la thèse que quelqu'un, que je ne dénoncerai pas, m'a tenue récemment (et presque ses paroles, d'ailleurs), même si j'ai complété avec une explication que quelqu'un d'autre m'a faite sur un sujet proche. Dans l'ensemble, considérez que toutes les idées intéressantes ne sont pas de moi et que toutes les erreurs, par contre, ont été ajoutées par moi. (Je trouve l'idée intéressante, même si je ne suis pas complètement convaincu qu'elle soit historiquement si correcte.)