Monergism

In the virginal conception God is fulfilling His promise to redeem His people by a work of divine restoration. What is in view is not "creation ex nihilo" but a new creation which is in the context of the old creation. The flesh by Mary weakened by the fall in which the Spirit operates in a way so mysterious that Scripture sheds no light on it whatsoever. Out of that humanity weakened by the fall, joining it to the persons of the Son of God the Holy Spirit sanctifies that human nature at the moment of His conception so that, that which is conceived in her is called holy, the Son of God. What then we have in the gospel testimony to the virginal conception is the notion that the Son of God enters into the closest possible union with humanity in its falleness in such a way that by the Spirit in the moment of conception that humanity is, by union with the Son of God, through the Spirit, sanctified in order that there may be in him the beginning of a new and holy creation. And the Spirit of God is however mysteriously active in this right from the very beginning of the human life of our Lord Jesus Christ. The Holy Spirit is present and active from the very inauguration of the life of Jesus so there is no point of Jesus existence as the Son of God incarnate in which the Holy Spirit is not already both present and active. This is one of the great mysteries of the Christian faith.
Sinclair Ferguson