If anyone disbelieves that Holy Mary is the Mother of God, such a one is estranged from the Godhead. Were anyone to say that [Christ] passed through the Virgin as through a channel and was not formed in her both in divine and human fashion--'divine' because without a husband's cooperation--'human' because conceived in accordance with human law-- such a one too is an atheist. Should anyone say that a man was made, and that God afterwards entered into him-- he is risking damnation.
Saint Gregory Nazianzen (325-389)
Further reading
Gregory’s insistence on Christ’s real humanity makes Mary’s motherhood part of a larger confession of faith. Mary: The Church at the Source offers more sustained theological reading on that relationship and on Mary’s place in the life of the Church.
Mary: The Church at the Source by Joseph Ratzinger and Hans Urs von Balthasar (paperback) on Amazon (paid link)