The First Reading (Ezek. 47:1-2,8-9,12) is interesting to me for a couple of reasons. First, Ezek. 47:1 is the origin of the Vidi Aquam, the traditional (and still used today, hopefully) antiphon used during the Rite of Sprinkling with Holy Water during the Easter season:
Vidi aquam egredientem de temploThe Rite of Sprinkling with Holy Water, in the Ordinary Form of the Mass, takes the place of the Penitential Rite at the beginning of Mass. In the Extraordinary Form of the Mass, this rite happens before the Mass actually begins (so it is not replacing, but supplementing).
a latere dextro, Alleluia;
et omnes ad quos pervenit aqua ista
salvi facti sunt, et dicent, Alleluia. Alleluia.
I saw water flowing from
the right side of the temple, Alleluia;
and all they to whom that water came were saved,
and they shall say, Alleluia. Alleluia.
The reading speaks of the east-ward facing temple (also a traditional feature of the great majority of churches). This text is also reinterpreted in the book of Revelation (cf. Rev. 22:2, for example).
Spiritually, the text speaks to me because this reading focuses less on the temple itself and more on the water which flows from it. This speaks to me of the waters of baptism, the regeneration in the Spirit which that sacrament brings. It speaks of this water affected everything it touches, which says to me that our baptismal rebirth is not simply a personal nor private event, but one which is ordered to ALL of creation. As we are created anew in Christ, so the world will be by us AND by Him.
I really hope my parish uses the assigned Responsorial Psalm (Psalm 46:2-3,5-6,8-9), since it is particularly appropriate. Jesus is quoting from Psalm 69, so that is also a relevant Psalm (perhaps one to meditate upon during your personal preparations for Mass).
The Second Reading (1 Cor. 3:9c-11,16-17) re-interprets the Old Testament reading for us as Christians. We are temples of the Holy Spirit, and as the Lord says in John 4:14, the water he gives becomes a spring within us welling up to eternal life. In other words, the baptismal water in which we are cleansed is the "water of life" which then grows within us as a spring, insofar as we are temples of God.
Paul also speaks of the importance of building upon the foundation with great care, and that the only foundation there can be is the one laid by Jesus Christ. We cannot try to build upon our old selves as a foundation: it is not permitted for us to fall back to our old selves or try to restore our old sinful ways and "christen" them. We are a new creation. We must be holy, set apart for God, not for impurity (cf. 1 Thess. 4).
The Gospel (John 2:13-22) shows us how Paul (who was once a Pharisee and a Jew with a mastery of their Scriptures) got from Ezek. 47 to what he wrote to the Church in Corinth: he was building upon the foundation of Christ's teaching: Jesus's body is the true temple, and we, being the Body of Christ, the Church, are ourselves temples of the Holy Spirit. Jesus also speaks of the importance of keeping the temple holy, of not profaning the temple.
Now, the feast day is about a physical edifice, so how does that fit in with all this "spiritual temple" stuff?
Church buildings should be constructions of stone and wood and glass and metal that represent the spiritual temple of our bodies. I think that is why the cruciform floor-plan gained popularity. Churches with tall spires reaching to the sky are not an exercise in vanity (like Babel, cf. Gen. 11) but a testimony to the transcendence and greatness of God. The bells peal forth an echo of the gospel, reaching farther than the eye can see. The sacred art inside, depicting our Lord, his Blessed Mother, and his saints, are pictures of our faith; they are models in our lives of faith.
The glorious beauty of a church building reflects the beautiful glory of the Catholic faith, of the Church, and of Her beloved spouse, our Lord Jesus Christ. It inspires us to holiness, fills us with awe, and reassures the hope that is in us. A church is truly a sacred place, a place consecrated to the Lord, as our very bodies and souls are.
That's what my birthday -- oops, I mean, the Feast of the Dedication of the Basilica of St. John Lateran -- means to me.