It's a question I feel like I haven't answered well enough in real life, but since I often find my thoughts clearer when written down, I thought I'd take a minute to write why I am a Catholic, and what that means in my daily life.
I'm a Catholic because if God exists and wants us to know Him, he'd have made it simple to do so. I avoid saying easy, because it's not always easy, but it is simple. Straightforward. If I didn't know whether God existed, and was investigating without any biases in the way, I'd probably want to find a religion that was well known, global. Universal. (the word Catholic actually means universal...it's not a brand like Coke) There are a handful of religions that fit that descriptor: Judaism, Islam, Christianity, Buddhism (sometimes described as not a religion, I know).
Judaism is wildly different since the Roman Empire scattered them and destroyed the Second Temple. There's an interesting coincidence between the ending of bloody animal sacrifices and the beginning of Christianity, which describes Jesus as the "Lamb of God". If God wanted to set the stage for an "ultimate and final" sacrifice that would suffice for the entire broken human race, it would look very similar to the way the Jewish people brought unblemished animals as vessels for their sins.
Islam was founded by a warrior. If God was very powerful, and wanted to show the world His power through a prophet, he would certainly have chosen somebody like Mohammed. Interestingly, the prophets in the historical tradition of the Jewish people were not warlords. Also interestingly, the further one got from Rome, the more heterogeneous the beliefs of Christians got. Some of the teachings of Mohammed on the Virgin Mary, and the Trinity, show that he based his reactions against Christianity on beliefs that the Church (and the Scriptures) didn't support. It's hard to believe a prophet who spoke as the penultimate prophet of God could be so wrong about such basic things. His marrying a 9 year old doesn't help either...
Christianity built western civilization. While it's not really taught in the American public school system, or most 100 level history classes at universities, scholars are pretty clear on this. Christians are responsible for hospitals, universities, the retaining of ancient knowledge from the Greeks and Romans, law, terraforming Europe, and so much more.
There are blemishes, too. Christ may be perfect, but His followers sure aren't. There's a habit, I think, for too much focus on that, to the detriment of an appreciation for the real benefits we have gotten from this tradition.
I'm a Catholic because it makes sense for God to have a teaching Church to guide people. It is more logical to have a final authority in difficult questions, rather than a book with millions of interpretations. The United States does not exist simply because of the Constitution, but also because people have a government that ensures it is followed. We argue about the meaning of articles written 200 years ago enough; it doesn't take much of a leap to realize that books written 2000-3000 years ago may be more complicated than most of us can understand without a very good teacher, and because people argue a lot, it makes sense for one teacher to be the final authority on such matters.
I'm a Catholic because of the beauty of the Church and her art.
Perhaps the greatest sculpture ever
This, too, by the same artist
Churches are beautiful.
Notre Dame in France. Beats any banks I've seen
The musical traditions of Western Civilization, from simple Gregorian Chant, to Bach's Masses , are incredible.
I'm a Catholic because it doesn't require me to disregard science. The leader of our Church is a trained chemist, and there is an embarrassment of riches when it comes to the intellectual and scientific tradition of our forefathers. Just start here
I'm a Catholic because regular Confession is free of charge and it requires me to look at myself honestly, without which I'd justify far too many of my proclivities. It is a font of mercy straight from God that gives me the strength to change myself for the better and the humility to know how fragile we all are. We have high standards, and lots of mercy.
I'm a Catholic because it requires me to love my fellow human beings without exception, even our enemies.
These are my beliefs; thank you for reading. I'm open to hearing yours in the comments-will you share what you believe?