We didn't cover it here, I don't think.
The Church as relationship with/within God begins before time with the internal fellowship of the Trinity. As far as humanity is concerned, it begins with Adam.
I would suggest it began with the creation of the first creatures, the noetic beings. The Church is the gathering of creatures in Communion with God. The word in Greek, which St. Paul uses a lot is ekklesia
, which means something like called out
Not a new coinage, but an old Greek word, which generally meant practically an assembly
So putting the etymology together with its quotidian playing out, we get something like: the assembly of those who were called out.
So I would suggest such a definition would only apply to those called out by God. I think there is a felicitous aspect of language at work. We were called into being. The Father was not. The Son is begotten. The Holy Spirit proceeds.
FWIW, the word qahal
, which certainly got glossed as ekklesia
, has a similar etymology and pragmatic upshot.
So this is just to say where I think the Church began and when, if such words can be used.
If we are in agreement here or close enough. Then we have to ask what is the ontological nature of the Church. I think my definition above is broad enough to begin: the Church is the gathering of creatures in Communion with God.
If we can agree this broad definition captures something (I am not sure how finely we need to dice for internets' and sanity's sake) of the ontological nature (I hate that turn of phrase) of the Church, then I think we can move rather forward in a reasonable manner to answering your question.
Let me know what you think.