Christ was "building up" his church that had been assembled around him since the baptism of John in the first chapter of the gospel of John (Acts 1:22-23). Paul says that God has set the apostles in the church first and they were already functioning and the office vacted by Judas was filled before Pentecost. On the day of Pentecost it was simply "added" unto not instituted (Acts 2:41). The three thousand were "added" to those assembling together as the Lord's Church in Acts 2:1 who previously had been assembling in Acts 1 where an organized church business meeting occurred under the leadership of Peter and by direction of the scriptures.
I understand Paul as saying in Galatians 3:9 to 4:7 that he --- a Jew --- first had to become a Gentile and like unto a Gentile in order to become a son and an heir of God through Christ.
Unless the Stone of Offence crushed one underneath, there's no hope he will stand up one day. "Are human beings different in nature from one dispensation to another?" not only in nature; in every way.
Ephesians 2:19-22 follows as day, night, the unity of the one Church of Jesus Christ, "...builded TOGETHER for an habitation of God through the Spirit".