What did Peter mean by calling believers a called out assembly in Acts 2?
Peter meant a called-out assembly of Jews who had separated themselves from the mainstream of Judaism by believing that Jesus was the Christ. He was not referring to being baptized into the body of Christ, because that revelation had not yet been given to him.
Les explains that Peter does not mention being baptized into the body of Christ at this point. Peter has not yet had that revelation and is unaware of it. Therefore, when Peter speaks of a called-out assembly, he is merely talking about an assembly of Jews who separated themselves from the mainstream of Judaism by virtue of believing that Jesus was the Christ.