Even when you give someone a well-constructed apologetic argument, it's always possible they will simply reply, "Being Catholic is your truth, not mine. It's true for you, but not for me." How should we respond to an assertion like this?
What is truth?
When we say a statement is true, we mean that it accurately describes the way the world is. Telling the truth means a person “tells it like it is,” or what they say “corresponds to reality.”[i] Saying “Catholicism is true” means that what the Church teaches is an accurate representation of the way things really are.
For example, God exists and created human beings with immortal souls. God sent his Son to die and atone for man’s sins. Christ founded one Church, with Peter holding special apostolic authority that was passed on to successors. To say that Catholicism is true is to say these things really did happen. When people say that the Catholic Faith is not “true for them” even though it may be “true for you,” they have confused two different kinds of truth claims: subjective truth and objective truth.
True for you
Subjective truth refers to feelings about the world, and the truth of those statements depends on who is speaking. So, when I say, “Ice cream tastes great,” that is true for me. Ice cream really does taste great to me. But it might taste terrible to you. If that were the case, then it would be true for you (but not for me) that “Ice cream tastes terrible.”
We can both be right, even if our beliefs about ice cream contradict each other. That is because we are actually not making statements about the nature of ice cream. We are instead making subjective statements about the feelings we have about ice cream. There’s no contradiction in different people having different feelings towards the same thing.
True for everyone
Unlike subjective truth, which relates to feelings about the world, objective truth relates to the world itself. Objective truths describe the world as it really is, and the truth of those statements does not depend on who is speaking.
For example, the statement “The earth is round” is true no matter who says it, because that is just the way the world is. Or, to return to our ice cream example, the statement “The ice cream is in the freezer” is either objectively true or it is objectively false. This fact does not change depending on how someone feels about the ice cream. For example, a yogurt fan can admit the ice cream is in the freezer even if he hates how ice cream tastes.
Universal (or Catholic) truth
Statements about the existence of God or the authority of the Catholic Church are objective truths. They describe facts about the world, and so they are true or false regardless of how we feel about those facts. When someone says Catholicism “is true for you, but not for me,” what he really means is that you think the faith is true and he does not.
You can say to him in response, “I respect your right to have beliefs that differ from my own, but shouldn’t we make sure our beliefs about the world are accurate? I think Catholicism accurately describes the world, and so that’s why I think it’s true. But if I’m mistaken, I’m willing to change my views. I’m even willing to believe what you believe, if you can give me a good reason to think that it is true. Would you be willing to change your views about the world if it turned out you were mistaken?”
[i] St. Thomas Aquinas used the expression, “Truth consists in the equation of mind and thing” Summa Theologica 1:21:2. Among modern philosophers this is also called the correspondence theory of truth, though it differs from the Scholastic view of just how the mind's ideas correspond to reality.