What does it mean that we have all been made to drink into one Spirit?
It means believers partake into the one Holy Spirit, who places them into the body of Christ. That body is the church, and it is not one member but many. This is the Pauline truth: the Holy Spirit baptizes predominantly Gentiles into Christ, a reversal from Christ baptizing Jews into the Spirit's power.
Les explains that being made to drink into one Spirit means partaking into that One Spirit, because the body of Christ is not one member but many. He notes that the church, which is His body, and the body of Christ, which is the church, is a term unique only to Paul. In the early part of Acts, Jesus is the baptizer, baptizing the Jew into the power of the Holy Spirit to perform signs, miracles, and continue Christ's earthly ministry. But when Israel rejected it and God turned to the Gentile, Paul completely flips the operation: the Holy Spirit becomes the baptizer and places us into Christ. Les calls this a whole, complete, 180-degree turn.