"God is love". This is the only place in the New Testament where this exact identification is made, and it appears here in the same letter twice. For a Catholic wedding, choosing this reading is choosing perhaps the most theologically dense single sentence in Scripture about what love actually is. Love is not a feeling God has; love is what God is.
The implications for marriage are profound. When two people love one another truly, they are not generating love from nothing; they are participating, however dimly, in the very life of God. Catholic theology holds that the Holy Spirit himself is the love between the Father and the Son, and that this same Spirit is poured into the hearts of believers. The love between you on your wedding day is, mysteriously, a created participation in the uncreated love of the Trinity.
John makes the moral claim explicit: "if we love one another, God remains in us, and his love is brought to perfection in us." The visible love of Christians for one another (and supremely, the visible love of spouses) is one of the privileged places where the invisible God becomes available to the world. Your marriage is meant to be one of those places. People who watch you across the years are meant to glimpse, in your love for each other, something of the love that God is.
If 1 Corinthians 13 describes what love does, this reading describes what love is, and the second is even more foundational than the first.
