Why didn't God expect Peter to understand the church age?
God did not expect Peter to understand the church age because He had kept it hidden as a secret in His own mind. Peter and the eleven operated only on the Old Testament program already revealed, having no concept of God turning to the Gentiles for roughly 1900 years.
Les teaches that the Church, the rapture, and Church doctrine are not to be looked for in the Gospels or the Old Testament, because that program was a secret held in the mind of God. During the early part of Acts, Peter and the other eleven apostles operated only on what God had revealed, which was the Old Testament program. Peter had no concept of God turning to the Gentiles for 1900 and some years, and none of them did. Les explains that this was not their fault; God did not expect them to understand it because He had been holding it a secret. God was still giving Israel every opportunity to repent of what they had done to their Messiah, and Peter was not talking to the Church or to Gentiles in those early chapters.