Paul reminds us that we are children of God. God's Spirit is within us. The idea did not originate with Paul. Israel spoke of God's Spirit dwelling within their community (Exodus 25:8). Paul recognizes that the Spirit lives within Gentiles as well.
Trinity Sunday is a good time to look at the people around you and consider what it means for you that God's Spirit is within each of them. And it's a good time to consider what it means for your congregation that God's Spirit is dwelling within your church body. What kind of witness are you viewing? What kind of witness are you showing?
(again, I'm thankful to Allen & Williamson's Preaching the Letters.)