Who was in the church in the wilderness with the angel at Mount Sinai according to Acts 7:38?

Les explains that Stephen is making a case to Israel that Jesus was the Christ. Moses had predicted a prophet like himself, and in Acts 7:38 Stephen says 'this is he,' speaking of Jesus of Nazareth, who was in the church—the Ecclesia, or called-out assembly of the children of Israel—in the wilderness with the angel. That angel spoke with him at Mount Sinai with the fathers, and he received the living oracles, that is, the Law. Les notes that when God called Israel out of Egypt, He brought them through a Red Sea experience that was a baptism without a drop of water. They became a church without any church organization, because the church here is a called-out assembly.