It’s important to remember that when you say “I’m certain that …” you are not talking about truth, you are talking about Psychology.
In church, you may hear your pastor ask if you know for certain where you’ll spend eternity. “For certain” is irrelevant to everything but emotion. It’s not the part that matters when it comes to truth; “Do you know” is the relevant question as “how” one can know isn’t purely relative or without requirements, like certainty is.
One cannot know that there is a Heaven or Hell or even deity. However, one can certainly have confidence in those things. Again, this is because there is no necessary tie between these being believed to be the case and them actually being the case.
The separation of truth and certainty is perfectly clear if one lists all the things which make them certain, confident, in their various beliefs. Some may include facts and others, experience, some aesthetics and some, science. The lists have no guarantee of matching and arguing the validity of one person’s list over another’s begs the question Philosophy has asked from the beginning but not answered: why that one? These are all things used to justify a belief, but aside from agreement and practicality, anything can justify any belief as long as of course there’s agreement or practicality about the means of justification. That is to say, they’re inseparable.
How we know anything, then, is because we have asserted something compared to which, nothing better or more suitable can be said, given our means of justification.
How can we be certain what we believe is true actually is the case? Because we’re happy with our justification, because we want our belief to be true, because we don’t like the alternatives, any number of reasons but none of them related to answering the question of how we know anything at all. In other words, certainty is relative. There’s no mystery then why we would feel certain about beliefs and mistake that for the actual truth about our beliefs. That’s because being a psychological matter, we feel they are true. And honestly, most folks don’t care about truth beyond that feeling.
Just a thought.