What is the flip side that the 'but' in Acts 1:5 points to?
The 'but' in Acts 1:5 points to the flip side of John's water baptism: the promise that the disciples would be baptized with the Holy Spirit not many days later. That future baptism with the Holy Spirit began at Pentecost.
Les explains that all through Scripture, when he shows the word 'but,' it signals the flip side. Here John baptized with water, but the Lord Jesus said the apostles would be baptized with the Holy Spirit in a future day, not many days hence. That promise was fulfilled at Pentecost. At this point in Acts, nothing had yet been said about going to the Gentiles with the gospel of grace; everything was still based on the Abrahamic covenant, dealing with Israel first and then the Gentiles.
For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.