For you were going astray like sheep; but are now returned to the Shepherd and Overseer{"Overseer" is from the Greek episkopon, which can mean overseer, curator, guardian, or superintendent.} of your souls.
Jesus is a cornerstone, the foundation upon which the church is built, but yet He is also a stumbling stone. The Jews stumble over Him because of the crucifixion. They could not understand the Messiah being crucified and so they stumbled over this stone. He became a rock of offence to them.