A cornerstone wasn't a key in an arch.
It is literally a stone placed where two walls meet.
The walls are compared to the believers from the Jews and those from the Gentiles,
the "strangers and foreigners".
The point was,
Jesus Christ brought the two flocks together into one fold by His Gospel of Grace,
placing both groups not under the Law,
but ordaining them to establish the Law.
19 Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God,
20 having been built on the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone,
21 in whom the whole building, being fitted together, grows into a holy temple in the Lord,
22 in whom you also are being built together for a dwelling place of God in the Spirit. Eph. 2