Galatians 4:26

But the Jerusalem that is above is free, which is our mother.

Abraham was a recognized type of God in the Old Testament, a type recognized by Christ himself in the parable of the rich man and Lazarus; and Sarah herself therefore bore a certain analogy as the holy bride, the church (the unity of God and Christ being pertinent to the analogy). As the sons of Sarah, Christians are upon a much higher level than the sons of the bondwoman.

Which is our mother ... There also seems to be more than a hint here that Paul was rejecting any notion whatever that the Jerusalem church was in any sense "the Mother church" in the earthly sense of that word. The "Mother Church" virus has afflicted all generations of Christians, notwithstanding the truth in evidence here that nothing "on earth" may in any sense be understood as "the Mother Church." It is likely here that one needs to look for the reason for Paul's refusal to deliver the findings of that church in Jerusalem to these very Galatians.