Do you honestly believe anyone could know the answer to this?
Yes.
Jesus said that they (the believers) would be endued with power from on high, and told them to tarry in Jerusalem to await the promise of the Father. They saw Jesus ascend on the Mount... After this, they went to Jerusalem and met regularly in the temple and praised God.
Obviously, they were believers that Jesus was the risen savior. Ergo they were believers (and thus saved). Ergo Sum, the body of Christ was already formed before the Day of Pentecost arrived. They Body of Christ therefore was not born on the Day of Pentecost. This is the inescapable conclusion...