Why is the book of Acts called a transitional book?
Its purpose is to bridge the change from God dealing with Israel to dealing with the Gentiles, and from Peter's apostleship to the circumcision to Paul's apostleship to the Gentiles. The gospel of the Gentile was committed to Paul; the gospel of the Jew was committed to Peter.
Les teaches that the book of Acts is transitional, not a complete break from Judaism and the Jewish Christians, but a bridging over from God dealing with Israel to dealing with the Gentiles. Les explains that the gospel of the Gentile was committed to Paul, as the gospel of the Jew was committed to Peter. He says you cannot show the gospel of the grace of God in Christ's earthly ministry or as Peter preaches in Acts. They are recognizing this great difference of operation. This is 20 years after Pentecost, and Peter has lost something, with James listed first.