When I was an atheist, my Evangelical friend (now a Reformed Baptist pastor) told me of a way I could know Christianity was true: It was prophesied, he said, that no one could walk through the Eastern Gate of Jerusalem except Jesus when He returns in glory.
I laughed at the notion but was quite pleased: all I had to do was go to Jerusalem, walk through this stupid gate, and Christianity is disproven! Now that’s the kind of prophecy I can get behind. One that can be demonstrated to be false.
Of course, I never went to Jerusalem to try it out, and soon I forgot all about it. A year later I became a Christian in the most unexpected way and soon learned about Protestant and Catholic differences. I wanted to prove Catholicism false (because I believed it was false, given what I had been taught in my Baptist church).
After talking with my Catholic friend, I learned about infallibility: the Catholic Church claimed that it taught infallibly on faith and morals and would never reverse a dogma. “Blood in the water!” I gleefully thought. “All I have to do is prove that the Catholic Church reversed a dogma or that one of its supposedly infallible teachings was false, and Catholicism would be proven to be a sham.”
I searched. I read. I tried to find one. And came up empty-handed. Sure, there were a few marginal cases from the past where this or that pope may have personally taught something erroneous. But nothing big and obvious and indisputable. And I knew that something as big and old and human as the Catholic Church must have reversed a dogma in an obvious way over the course of two thousand years. Or at least teach a falsehood.
So I focused on the teachings. Contraception in particular. “Who on earth could be against contraception?” Remember I was an atheist my whole life; contraception was unquestioned. As a Protestant, I never heard anything against it, either. So I studied the subject, waiting to laugh at the ridiculousness of Catholic doctrine on it. But as I read, I didn’t laugh, and the smile was quickly wiped off my face. “These arguments are pretty good,” I thought. “What else is there to this teaching?” And I kept digging, and the more I dug the more it made sense.
I presented this information to my Protestant friends and played “Catholic advocate” as we debated it. They didn’t have good counter arguments. They mostly thought I was crazy for even questioning whether contraception was wrong. That was an important milestone in my journey to Catholicism: on the doctrine I “knew” was most patently wrong, I had been the one who reversed my beliefs on the subject, contrary to everything I had expected. There was something to this Catholic thing, and I had to investigate further.
The rest is history. I never found a reversal of dogma. All the doctrines I examined had solid grounding in Scripture and/or Tradition. They were also good reasons supporting them. So I became Catholic.
I don’t put much stock in no one being able to walk through this Eastern Gate before Jesus, but I do put stock in the Spirit guiding the Church, leading her into all truth and reminding her of all that Jesus taught.