Reason # 1: The Church is gritty, physical and real, and not some sort of abstract, extrinsic reality.
The material world matters, hence why the Church uses concrete elements like water, oil, bread and wine as means to convey the divine: “If the Church is real, she must be an organism that we can in some sense see and touch, just as we could have seen and touched the God-Man during his life on earth” (De Lubac, Splendor of the Church, p.87).
Reason #2: The Church is by which, and in which, we are united with Christ.
The Church is the “people of God”, because the entire human race has been assumed by Jesus Christ, whose presence is continued by the Church: “For the Word was the bridegroom, and human flesh the bride…The Womb of the Virgin Mary, in which he became head of the Church, was His bridal chamber” (St. Augustine of Hippo, Tractates, 8, 4).
Reason #3: What’s a King without a Kingdom?
The Church does not simply proclaim the kingdom of God, or point beyond itself to that divine reality, but the Church makes present that to which it points; the Church is the coming of the “kingdom of Christ now present in mystery” (LG, 3)
Reason #4: There’s no such thing as “lone-wolf” Christianity.
Faith is at once personal and ecclesial: ”no one can have God as Father who does not have the Church as Mother” (St. Cyprian, De unit. 6: PL 4, 519)
Reason #5: The Church matters, because the Eucharist matters!
The unity Holy Communion presents is a big thing for us Catholics, because not only is it communion with our Lord, it also communion with each other– with the Church– as the Catechism states, the Eucharist makes the Church (CCC, 1394).