I was an atheist until I was 18, but Catholicism always presented itself as the only live option for me among the Christianities: or, at least, as Evelyn Waugh put it, it just was Christianity. My reasons were more or less those of Cardinal Newman, though I hadn't read him at the time. In brief, without a certain kind of divinely guided tradition, you can't get creedal Christianity out of the Gospels. The principles other denominations would like to apply to secure their own doctrines are either over- or under-inclusive; they either don't suffice to yield the denomination's doctrines, or yield some of Catholicism's doctrines when they are applied consistently.
I couldn't, of course, put the argument very sharply, and I couldn't now. I'm not an expert in early Church history.