I was also recently asked by another friend the following two questions:
"What keeps you with the Catholic Church? What is the biggest thing that Catholicism offers that convinces you that the Catholic Church is *the* Church?"
These questions can be answered more quickly, so it might be worth setting out in brief. I'll begin with the second part of the question and then move to answering the first part. So what does the ancient Church offer us?
As an ascetic and a mystic, I have learned that to love truly requires us to kill the egotistical thinking, feeling, doing, and willing which so saturates our lives. It has been my experience that the Catholic Church offers us the most powerful set of spiritual practices and anthropological truths which help us to kill those egotistical modes of being and open us up fully to living our lives in the light of divine love.
This is not to say that other traditions do not also have practices and anthropological truths that help us to do that; the Orthodox communions, the Evangelical churches, various Buddhist schools, Taoist sects, Hindu traditions, Islamic traditions, Rabbinical traditions, and so on certainly have some which are powerful in that way. It is to say that my experience has been that the Catholic Church has a superlative set of these practices, that the value of each of those traditions is matched or improved upon by those in the Catholic Church.
This is of course going to sound like a bold claim to some, and they may want evidence. That's completely understandable, and I plan to provide it. That's just going to require a very long book, so you may have to wait until I'm retired and have the time to write it. So let's move on to the first question.
What keeps me in the Catholic Church is that I have found nothing better. That is not for lack of trying, by the way. I have studied Buddhism through the Pali canon and sincerely tried to adopt its practices. I have read the Koran and prayed with Muslims. I have read the Tao Te Ching and sought to reach a state of wu wei. And that's only the beginning of my religious exploration. Lest non-religious folks feel left out, I have read compelling atheist thinkers like Nietzsche, Bertrand Russell, and Sam Harris; I treat atheism as a serious intellectual option.
Closer to my current religion, the Eastern Orthodox communions are an appealing option. Their practices are almost identical to Catholic practices, and from a doctrinal standpoint, I can honestly profess everything in the Creed used in their liturgies. Not only that, but I also tend to gravitate more to Eastern theology and in some ways prefer their liturgies to the one I attend each Sunday. This of course does not change my decision, because my preferences do not tell me what is objectively true; they simply tell me about my preferences. In order to be chrismated in an Eastern Orthodox church, I would need a compelling truth-related or practice-related reason for making that move, and I have not found one yet.
Everything which is good, true, and beautiful that I have found in other religions or in the Eastern Orthodox communions (whether in part or in whole), I have also found in the Catholic Church. This state of affairs might change; after all, I have only been doing my exploration for ten years in my spare time. That said, for the time being, I'm still Catholic, and faithful to the Catholic Church as I can be.