Can we know anything about the external world?

Yes, but you seem to not just believe in those axioms, but act based on that belief. You don’t seem to be against what @Arzin28 wrote at all

Because I function better when I operate with certainty. I do not like doubts, it is not my style, that is why I prefer an Epicurean type philosophy.

Materialism gives more certainty, agnosticism is full of doubts.

I have searched a lot before deciding what suits me better, it is not that I started as materialist.

You like more when having certainty, but you can be certain of false things.

You are just assuming you ‘operate better’. Having certainty in false things is worse than being uncertain about true things

That is your opinion.

A religious person in my opinion believes in several wrong things. But these beliefs bring certainty in his/her life, has a happy life and enjoys it. What is the “worse” on that?

Fair enough, so: is it better to have certainty in false things or to be uncertain about true things?

Do you think certainty and happiness justify its means?

You seem to be confusing religious people with people in systems of belief. They are alike but not the same necessarily

I gave an example. I could have been more precise and say Christian.

Whatever suits anyone. I will not impose rules, because life taught me that there is no single philosophical or religious path that works for everyone.

What does christianity have to do with knowing anything about the external world?

I think you still have to address @Arzin28 ‘s points