Paul calls the Church the body of Christ, defining it as "the fullness of the one who fills all in all." Here, as can be seen, he follows traditional Jewish ideas about the people of God as one spiritual whole, as though one body, which was sanctified as one whole through the worship services that regularly took place in the Jerusalem Temple until its destruction in AD 70.
It was precisely because of this sanctification of the people as one whole that even those believing Jews who could not get to Jerusalem for a feast, for example for Pesach or Shavuot, did not feel cut off from God or from their community. They gathered where they lived, in the synagogues of the diaspora, and celebrated together with those who were then in Jerusalem. Paul applies this view to the Church, with the difference that it is sanctified by the presence of the Messiah himself, risen from the dead and abiding at the throne of God.
Of course, the Church is the space of the Kingdom abiding in our world as it is being transformed, though it has not yet been transformed completely. But it is also the space of Christ himself, the space of his personal presence. Certainly, God's presence had been known to the Jewish people before. God revealed himself to them already when the Torah was given at Sinai, revealing himself so that he would no longer be hidden, and his presence accompanied the people throughout their history.
But now the space of those relationships that bind Christ with his heavenly Father has been added to the spiritual space of this presence. The Kingdom unfolds within these relationships; it is revealed in them and through them. Their fullness is what fills "all in all." And Christianity means being included in this fullness of relationship between the Father and the Son. This kind of inclusion can be called true incorporation into the Church in the proper sense of the word, while everything else in church life is only a means for solving this main task. That is, if we are speaking precisely about the Church with a capital C.
