This is something I struggled with for a long time, and epistemologists I believe are still very divided on the subject.
Let’s say you have a statement that you believe is true. How about “God is real. ” Now, in order to say that it is true you must show why you believe it is true. So, you list out all these arguments. But then you must prove that those statements are true. And then you must justify your proof of your arguments. And so on, ad infinitum. So, there are now three options left to you to find certain knowledge.