"How can I [understand it] unless someone explains it to me?"

said the Ethiopian eunuch, a sentiment echoed here:

...What's that? You can read a map all by yourself you say? You don't need any help reading maps? Well, I would really like to believe that about you but my own experience has been different. I almost never get lost, geographically speaking. Just ask my wife. And I've spent an awful long time in the map room too and I love reading maps as well. But in my practical, real world experience of actually navigating out in the field as a Marine? I know that some people read maps wrong. Dead wrong.

And they were reading the same maps that I had, too. I can't even remember how many times I have had to point this out to lost Lieutenants, Captains, and sometimes even Majors, when I was out in the field in the Marines. And to PFC's, Lance Corporal's, and even Sergeants sometimes too, as they were learning land navigation skills. And this assumes you are using current maps that were drawn and printed recently. True story time. This may shock you, but I even knew a Captain in my artillery battery who got lost routinely(!) even when he was using GPS. I kid you not! So don't argue to me that the latest technology will absolutely guarantee that you will make it to your intended destination. 

Now, what if the map you are using today is ancient? You know, like you are using one that looks something like Blackbeards treasure map, or the one from Robert Louis Stevenson's classic book Treasure Island. You can see that there is an X that marks the spot of the treasure but not much more detail than that.

Well, if I were you, and I found a map like this, I would track down and find the guy who buried the treasure who, as it turns out, is also the same guy who drew the map, and I'd say,

Lookee here, I can't make head nor tales of where in the world this here treasure is from a readin' your map all by myself. Show me how to read this map and take me to the place where "X" marks the spot.

That is where the Catholic Church comes in see? She made the map, and she knows where the treasure chest is. Sure, I can read that Treasure Island map too, but it's lacking in a few details, or didn't you notice? How long have you been reading that map and you didn't notice this?! Now, the Church knows where the treasure is buried, because She was there when the chest was put into the ground. And She was there when it ascended up into Heaven to.

She knows that the treasure resides in each and every one of us now, so the map isn't a geographical one, see, but an internal one. As G.K. Chesterton explains so well,

The Catholic Church carries a sort of map of the mind which looks like the map of a maze, but which is in fact a guide to the maze. It has been compiled from knowledge which, even considered as human knowledge, is quite without any human parallel.

There is no other case of one continuous intelligent institution that has been thinking about thinking for two thousand years. Its experience naturally covers nearly all experiences; and especially nearly all errors. The result is a map in which all the blind alleys and bad roads are clearly marked, all the ways that have been shown to be worthless by the best of all evidence: the evidence of those who have gone down them...