You can be 100% certain that God exists. Everything else you can be so sure.
I am not backpedaling at all. We all have knowledge of good and evil, it is perspective we lack. God cannot be completely known, because to completely know God would be to completely know everything. Only God knows the thoughts of God.
You can certainly have knowledge about things. You clearly know English at least well enough to hold a conversation, and it would be silly to deny this.
The point isn't being 100% certain about stuff. You don't revert into a solipsistic black hole because you can only be 98% sure of anything. That is like treating dandruff by decapitation.
The point is that you want to be more honest, more sincere, and strive for reality. It is silly to even dispute that this isn't beneficial. But truly, it is best to love The Truth for the sake of it in itself than to do it for some type of gain. Gain is not godliness, and in fact, this in itself is an idol before God. To take gain as a reason to love The Truth actually gets in the way of loving The Truth.
In fact, some of the most brilliant people love The Truth as it relates to their gain. This is wickedness though, and a corrupted form of Truth worship.