Lumen Fidei chapter 3
There is a lot of truth that we accept that is simply handed to us. If I tell you that I typed this post on my computer and posted it, though you have no way (or even if you did you wouldn’t bother) of testing and knowing for fact the veracity of this supposed truth, you would most likely accept it for any number of reasons. We also accept all sorts of other things because we are part of the community of man. We receive our name. We know a spoon is called a spoon and not a dump truck. Community stores this collective knowledge and passes it on. That is why we are Church and not a collection of “Me and Jesus” people.
We are most Church when we celebrate the sacraments and follow the teaching of our Church, which is kept safe for us through apostolic succession and Tradition. When we step outside of these things we run the risk of losing truth (or truth becoming relative) and the unity of man (and man with God) is in jeopardy.
If this intrigues you, I invite you to read the whole thing which is much deeper than this little synopsis.