I think it was Dorothy Sayers who, speaking of the most blessed, holy, and undivided Trinity and quoting initially the Athanasian Creed (Prayer Book, page 864) wrote:
The Father incomprehensible, The Son incomprehensible, the Holy Ghost incomprehensible- the whole darn thing incomprehensible!
I wonder when you would say the Church was born?
Perhaps you would suggest: The call of the twelve disciples? The start of the community of the "sent ones."
At the foot of the cross, when Jesus commends St. John to his and our Mother and her to the Beloved Disciple? - a community of love rooted in the sacrificial love of the Cross of the Lord himself.