Why did Stephen call Israel "the Church in the Wilderness"?
Stephen called Israel the Church in the Wilderness because Ecclesia means a called-out assembly. Israel was called out of Egypt and assembled by God around Mount Sinai, making them an Ecclesia. This was not the Body of Christ, but a general assembly. Stephen used the Greek term in its broader sense.
Les explains that the word Ecclesia, from which the word Church is translated, is used in different manners and ways than just the Church, the Body of Christ. Stephen, in chapter 7, refers to the children of Israel out in the wilderness around Sinai as the Church in the Wilderness. Les points out that this was that called-out assembly in the wilderness. That makes sense because Israel was called out of Egypt, God assembled them around Mount Sinai, and so they were an Ecclesia, called the Church in the Wilderness. Les emphasizes that this is not the Church, not the Body of Christ. He also notes that in Acts a riotous mob in Ephesus was called an Ecclesia, though the translators fortunately called it an assembly. Therefore, the word Church does not automatically mean the Body of Christ, because Paul almost always qualifies the present-day Church as the Body of Christ.