In these last days Christ took a soul and body from Mary. It is this flesh that he came to save, that he did not abandon to the underworld: he united it with his own spirit and made it his own. This is the marriage of the Lord, united with the flesh of man, a mystery uniting the two — Christ and the Church — in one flesh.
From this marriage and from the coming of the Spirit of the Lord from above, the Christian people is born. The substance of our souls receives the seed of heaven: we are conceived in the womb of our mother and born of that womb we receive life in Christ. So St Paul says, The first man, Adam, as scripture says, “became a living soul;” but the last Adam has become a life-giving spirit. It is through his priests that Christ sows his seed in the Church. St Paul, again, says: It was I who begot you in Christ Jesus. It is the seed of Christ, that is, the Spirit of God, that produces the new man through the priest’s hands, conceived in the womb of his mother and born in the baptismal font under the auspices of faith.
We must receive Christ so that he can give us birth, as the Apostle John says:
To all who accepted him he gave power to become children of God.
But this cannot be brought about except by the sacrament of cleansing and anointing, the sacrament which the bishop administers. Sins are washed away by the cleansing waters of the font; the Holy Spirit is infused by oil of chrism; and we receive both at the bishop’s hands and through his words. Thus the whole man is born again and made new in Christ: so that as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. That is, having put behind us the errors of our former life, we should through the Spirit follow a new way of life in Christ.
Source: The Liturgy of the Hours – Office of Readings
From a Sermon of Saint Pacian on Baptism