I hadn't thought of that before, that Mary was the first member of the church - the invisible church that is. Orthodoxy says that she was entirely sanctified at the Annunciation where the angel of the Lord called her 'full of grace' (not from 'The Immaculate Conception').I'm not sure we need to nail down when the church began, in the sense that we don't need to worry about it or make it into a doctrine. Strictly speaking we might say it began with the first advent of Christ. He was the Head of it and began bringing in "members" as soon as He began preaching. And maybe we might even consider that His mother was the first member since she was seen having faith in Him at the wedding of Cana before Jesus had begun His ministry as such.
But I think it is very important that we know when the church started - the visible one that is, which gives a witness to the world, even though it consists of wheat and tares, as we need to know which are the cults, false churches, that do not teach holiness doctrine showing The Way to the deification of man which is the restoration to that to which he lost at the fall. Sadly in these dark times, it is now not even in the original church as they have mainly succumbed to the 'gradual sanctification' of worldly churches whereby holiness is not obtained till death.
Re cleansing, on meeting a man walking in the Spirit and led by the Spirit, people will be convicted of their sins even before he speaks as in the story about Charles Finney and the factory girl who came to Christ just looking at him after she had been making jokes about him.
When I was fully walking in the Spirit, I attended a Bible study group where one woman was seriously convicted by what I was saying, but the leader of the group soon put an end to it by telling her that she was stressing herself un-necessarily as even her future sins had been forgiven. He won.
When we first come to Christ, we are very sensitive to sin and will try desperately to avoid it, but soon we will begin to compromise if we have not gone on to perfection, and then we 'lose our first love' and remain so unless we are revived through the first stage of entire sanctification.