This is one of my favorite passages because it speaks of the heart, or innermost being, as a wellspring of spiritual life. In just a few words Jesus tells us what it means to be born again. Many argue about the theology of salvation and seem to skim over this aspect. I once heard a pastor put it this way: No Jesus = No Change; No Change = No Jesus. There is a symbiotic relationship between believing in Jesus and a changed life. Yet some do not understand this dynamic of a heart radically changed and reborn by God's Spirit.
In the last sentence of our passage John tells us that Jesus was alluding to the coming day of Pentecost when the Holy Spirit would come in an amazing and powerful way. As he writes the apostle looks back to that day when he himself was radically changed. In hindsight he understands that Jesus was speaking of how the Spirit would be poured out on those praying in the upper room. And that is the message in these words - when we believe in Jesus the Holy Spirit comes into our lives and radically changes us from the inside out.
Come Holy Spirit. I need you today. I yield. Baptize me afresh.