How do we know that what the Church teaches is true? Well, how do you know that Germany has a population of ca. 80 million? (If you didn’t know that before, well, you do now.) Even better, how do you know that in 1492 Christopher Columbus undertook a voyage that led to his landing in the Americas? My guess is that you’ve never done an eye-witness head count of the entire German population and that you were not around in 1492 to accompany Columbus on his voyage. But in the 15th century Columbus did come back and tell of his travels, and he had other witnesses to back him up. Lots of other people, believing all these witnesses to be credible, took thier testimony as fact, some recorded it. That line of witness continued throughout the generations of humanity until you were informed of the fact by the most recent link in the chain of human testimony. When it boils down to it, the only “proof” you have of Columbus’ expedition is that someone trustworthy told you it was true. The same probably applies for the veracity of the theories of relativity, the distance of the sun from the earth, and the age of the United States of America. You don’t remain agnostic about Siberia just because you never been there.
What’s the point of all that? Well, despite the fact that we sometimes do conduct our own experiments to “prove” current states of affairs, most of our knowledge is derived from testimony of other people. We don’t really “know” these things so much as we “believe” what we’ve been told. That’s one of the main reasons we believe in the Truth taught by the Church. The Apostles and many others were witnesses to Christ. They testified to his teachings and deeds, and, since they were credible, many people believed them. Through time, those teachings have been passed from credible witness to credible witness. If that’s the same standard of proof you use for 99.9% of your “knowledge”, why create a double standard for matters of faith?
Obviously, once one has accepted that one has to figure out, amid all the supposed witnesses of today, who is reliable. But thanks to the fact that witnesses throughout time have left writings, one can check current testimony against all the written testimony passed on in the chain of witness. This will back up the witness of the Catholic Church.