The 'Word became flesh' (Jn 1: 14)
The particular question of the moment at which the soul is created one with the body, is asked in the context of understanding the works of God through the doctrine of the Incarnation of Christ.
This is because the creation of the soul and the creation of the soul in union with the body, is first and foremost an act of God.
Therefore, considering the beginning of human life from the perspective of it as an act of God (cf Gn 4: 1), it would seem that there is a distinct possibility that the doctrine of the incarnation will be relevant to understanding the act of God which is at the beginning of each of us.
The possibility that the doctrine of the incarnation is relevant to understanding the beginning of the life of a person is as it were present in an acute form in the following two quotations.
The first quotation is from Gaudium et Spes: 'The Word of God, through whom all things were made, became man and dwelt among men: a perfect man, he entered world history, taking that history into himself and recapitulating it'374.
The second quotation is from Familiaris Consortio: 'As an incarnate spirit, that is a soul which expresses itself in a body and a body informed by an immortal spirit, man is called to love in his unified totality'375.
On the one hand the event of the incarnation of the Word of God is a wholly unique event, precisely because the Son of God 'has existed from all eternity'376; on the other hand this does not seem to preclude the possibility that it is yet capable of containing within it the mystery of human conception.
Not the human conception that requires the agency of a human father, although the absence of this makes present the Fatherhood of God the Father, but the nature of human conception from the point of view that the creation of the soul and the soul's union with the body is necessarily an act of God.
In other words, there seems to be no discernible reason why the conception of the soul of Christ and its simultaneous and immediate union with the body, is not itself a recapitulation of the history of each one of us.
The fact that the coming into existence of the body of Christ is an event which did not require the agency of a human father, is a fact that distinguishes the creation of the body of Christ from all other human beings except that of Adam and Eve; however, this fact does not disqualify the identity of the mystery of the creation of the soul of Christ and its union with His body, with that same mystery which is at the origin of all human beings, including that of Adam and Eve.
Therefore, while there may be other reasons why a person may wish to argue that Christ's soul at conception is different with respect to that of other human beings377, it does not follow from these exceptional characteristics of Christ's soul at conception, that the simple fact of being given a soul at conception is itself exceptional.