It was because God was felt as a Father that the belief in the Trinity arose. For a God-Father cannot be a single, that is, a solitary, God. A father is always the father of a family. And the fact that God was felt as a father acted as a continual incentive to conceive Him not merely anthropomorphically--that is to say, as a man, _anthropos_--but andromorphically, as a male, _anêr_. In the popular Christian imagination, in effect, God the Father is conceived of as a male. And the reason is that man, _homo_, _anthropos_, as we know him, is necessarily either a male, _vir_, _anêr_, or a female, _mulier_, _gynê_. And to these may be added the child, who is neuter. And hence in order to satisfy imaginatively this necessity of feeling God as a perfect man--that is, as a family--arose the cult of the God-Mother, the Virgin Mary, and the cult of the Child Jesus.
The cult of the Virgin, Mariolatry, which, by the gradual elevation of the divine element in the Virgin has led almost to her deification, answers merely to the demand of the feeling that God should be a perfect man, that God should include in His nature the feminine element. The progressive exaltation of the Virgin Mary, the work of Catholic piety, having its beginning in the expression Mother of God, _theotokos_, _deipara_, has culminated in attributing to her the status of co-redeemer and in the dogmatic declaration of her conception without the stain of original sin. Hence she now occupies a position between Humanity and Divinity and nearer Divinity than Humanity. And it has been surmised that in course of time she may perhaps even come to be regarded as yet another personal manifestation of the Godhead.
And yet this might not necessarily involve the conversion of the Trinity into a Quaternity. If _pneuma_, in Greek, spirit, instead of being neuter had been feminine, who can say that the Virgin Mary might not already have become an incarnation or humanization of the Holy Spirit? That fervent piety which always knows how to mould theological speculation in accordance with its own desires would have found sufficient warranty for such a doctrine in the text of the Gospel, in Luke's narrative of the Annunciation where the angel Gabriel hails Mary with the words, "The Holy Spirit shall come upon thee," _pneuma agion epeleusetai epi se_ (Luke i. 35). And thus a dogmatic evolution would have been effected parallel to that of the divinization of Jesus, the Son, and his identification with the Word.