"The Angel of the Lord declared unto Mary.
And she conceived of the Holy Spirit." (Angelus)
At the Annunciation, Mary conceived Christ. A union between God and Man took place in her womb, thus at Conception, Christ became the true mediator between God and Man because he was both "True God" and "True Man". However, Christ achieved the fullness of his priesthood with his Death/Resurrection and Ascension.
Christ: Faith and Hope