What does Les mean when he says Acts 2 is a furtherance of the same message?
It means Acts 2 continues John the Baptist's call for repentant Jews to be baptized, now grounded in Christ's finished work and shed blood. Yet it is not the later Gospel Paul defines in 1 Corinthians 15—Christ died for our sins—because the message says, 'You killed Him.'
Les explains that John the Baptist baptized repentant Jews, creating a separated group even then. In Acts 2, the same thing happens: about three thousand gladly received the Word, were baptized, and were added to the believers. The furtherance is that the finished work of the cross has now been consummated, so God could save these people based on the shed blood. But Les stresses that this is not yet the Gospel he defines from 1 Corinthians 15—that Christ died for our sins. Instead, the message was 'You killed Him.' The twelve apostles still knew mainly that Christ had come and fulfilled all the Old Testament promises.