AB,
I know evil exists because I know good exists.
You can't know one without the other.
No, what you “know” is that it's observably the case that desirable and undesirable events occur. “Good” and “evil” in the personified religious sense you’re attempting is just a narrative you find persuasive, much as child might who runs into a branch might say “that tree hit me”.
You will never understand my certainty from outside.
"Knock and the door will be opened unto you." Matthew 7:7 (from the sermon on the
mount)
First, you’re just repeating your expression of faith without making an argument that would justify the claim.
Second though, as your only purpose here seems to be to evangelise (albeit that you’ve picked the wrong demographic for that) then why try it with beliefs that, apparently, can’t be “understood from outside”?
Tell you what, why not try the question that Vlad always ran away from a while back no matter how many times it was asked: if I lined up ten people before breakfast (nine plus you) and each of them told me with equal conviction they were certain of their different beliefs that were written in “holy” books, why should I take your assertions any more seriously than I should take those of the other nine?
You cannot measure or verify faith in the way you are suggesting.
Yes, we are all fallible - but there is a basic fundamental truth which will enlighten us to the meaning and purpose behind our existence, and you will know it when you find it.
But what you’re actually being asked to justify here is your claim that you “know” something rather than just happen to believe it to be true. Why can’t you do that and, as you apparently can’t, why in that case should anyone take your claims and assertions seriously?