Can anyone be a father without beginning to be one? Yes, one who did not begin his existence. What begins to exist begins to be a father - God the Father did not begin at all. He is Father in the true sense, because He is not a son as well. Just as the Son is son in the true sense, because He is not a father as well. In our case, the word 'father' cannot be truly appropriate, because we must be fathers and sons.
Gregory of NazianzusMore Gregory of Nazianzus quotes
If anyone does not believe that Holy Mary is the Mother of God, he is severed from the Godhead. If anyone should assert that He passed through the Virgin as through a channel, and was not at once divinely and humanly formed in her (divinely, because without the intervention of a man; humanly, because in accordance with the laws of gestation), he is in like manner godless.
Gregory of Nazianzus