Since that answer is probably not too useful, I'll flesh it out some, recognizing that the full answer is literally beyond any human comprehension.
(As a relevant aside, to say that something is beyond our understanding is not a "cop out" or an aggrandizement, as some seem to think, but is rather a simple statement of reality. There are simply some things -- in fact, a lot of things -- that we cannot explain because we have finite minds and hearts, and God is infinite.
One good working definition I've found of that slippery virtue of humility is that it is the recognition of the truth about ourselves and about God -- that we are flawed and He is perfect. So it is truthful and humble to acknowledge that there are things that cannot be explained.)
In a way, this whole blog is a fuller answer to the question, but here's the bottom line.
We are Catholic because:
- In love, God made us and sustains us, AND Jesus, the Second Person of the Trinity (God), established the Catholic Church to be His Body on earth, AND Jesus made clear His desire that we be part of Her. The New Testament (part of God's Word) overflows with references to the Church (just pick a page and you're almost certain to find at least one reference to Her). One of the best Web pages explaining this and so much more is Catholic Answers.
- The Church has stood the test of time, just like Jesus said it would ("And I say to thee: That thou art Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it" [Matthew 16:18].)
- We were blessed to be raised in the Church and -- in more than three decades of living in the world -- have never seen anything else that approaches Her truth and beauty, most especially in the Mass, the Eucharist (Jesus' Body and Blood, His greatest gift to us!) and the other sacraments. We love Her!
We are a family because:
- Through a mind-boggling array of circumstances over many years (indeed, from before time began), God created my husband and me for each other and, at the perfect moment, brought us together in marriage.
- Furthermore, at exactly the right moments, He used our marital sacrament to bring forth four children. It is likely that He will do so again.
- We are, then, the "domestic Church."