I am Catholic because Catholicism is true.
It is not a little true.
It is not some truth mixed with error; if I wanted that, I definitely wouldn’t be here. I am Catholic because the Catholic Church is the only place you will find the fullness of Truth. It is for Truth that I became a Catholic, and it is for Truth that I will die a Catholic.
At the start of my conversion, I did what, as far as I was concerned, was the only logical thing to do for someone who thought that God might be saying to him: “Become a Catholic”. I made a list of everything that I thought was Not True about the Church. I listed every teaching I disagreed with. And then I set to work. I began to listen to the real arguments for those teachings, and the real arguments against them. I weighed everything I was unsure of in the balance.
And what I found was that, one by one, I crossed items off the list.
I was not about to become Catholic because of some feeling. I was not about to become Catholic because I was escaping something. I would become Catholic only if I was convinced that it was true in all that it taught. That was the only thing that mattered, for me: Is this true?
I almost got there that way. There came a point when I had solved most, but not all, of my difficulties with what the Church taught. I was still not convinced about contraception. I was still not convinced about Mary. I was still not convinced about the rejection of sola scriptura.
But one thing I had solved was the authority of the Church to teach these things. I knew that the Church was protected by the Holy Spirit from ever teaching error. And so I said to myself: Well, if the Catholic Church can not teach any doctrine that is false, then any remaining problems that I have are my own error, and not the Church’s.
That was a key moment for me: the realization that I am not the arbiter of Truth. The Church is, guided by the Holy Spirit. I am not the Church’s teacher; the Church is my teacher.
That is why I am Catholic, and will always be Catholic. If I go anywhere else I will be left to my own wits. And I can’t make it by my own wits. If there is anything I have learned in 46 years, it is that I can’t make it by my own wits.
C.S. Lewis puts it like this in Mere Christianity:
And above all, you must be asking which door is the true one; not which pleases you best by its paint and panelling. In plain language, the question should never be: “Do I like that kind of service?” but “Are these doctrines true: Is there holiness here? Does my conscience move me towards this?”

Peter Kreeft, commenting on Lewis, has this to say:
Now, that sounds very simple. But it’s the essential question to ask in making any honest choice. There’s only one reason to honestly believe anything: because it’s true. If you think differently than that, let’s get that settled before we do anything else.

Their reasons are my reasons but what do other say?
People stop being Catholic for all kinds of reasons. Some people leave the Church because a priest said something mean to them, or treated them in a shabby and unwelcoming way.
Some people even leave the Church because the art isn’t good anymore.
I’m sorry, but that’s just nonsense to me. You know what? I wish no priest ever said a mean thing, ever. I wish every Catholic artist were Michelangelo and every Catholic parish looked like the Cathedral of Chartres. But what do you expect to find when you leave the Church? Is there some religion out there where the clergy are all saints and none of the buildings are ugly? Please. If you’re leaving the Church because of that, you’ll find it’s all the same on the outside.
But what you will find different, outside the Church, is that there is a whole lot of error and a lot less truth. That’s the one thing you are guaranteed to find different outside the Church. Leave if you wish, but I’m not leaving with you.
When I became Catholic, I walked on air for a year.
And then God took that away from me.
The years since I have become Catholic have been among the worst of my life. I have lost four different jobs. I have been suffering from a depression I can not see the end of as far as I look. I go to Mass, I go to Adoration, and everything is dry bones.
I wish I could say otherwise, but I must tell the truth about this. The Catholic Church does not promise that everything will be easy. It promises, instead, a Cross. Fr. George Schommer, who preached the closing homily for my RCIA class the year I became Catholic, said it, “You will be tested.”
And yet, I have never once been tempted to go and leave. I have never once said, “You know what, I was happier when I was a Presbyterian, let me go back there.” As though I was happy then because I was a Presbyterian, or as though the Christian life is about our happiness, or our feelings.
No. It is about Truth. It is about only Truth, for Christ is Truth, and in Him is our salvation.
This is important. If everything were about our feelings, then none of us would stay. Fr. Grumpy says something mean in confession, or the Cathedral Parish of Silly Art grates our aesthetic sensibility, and we’re off—where? To a lie? To our own wits?
No. Here I am, and with God’s grace here I stay, no matter whether the cross will be eased or pushed down heavier on my shoulder. I gave up everything I ever knew to follow Truth, and Truth alone, and I’m not turning my back on it. Not ever.