At least as early as my days at OBU, I recall smart-aleck fools who insisted that Ecclesiastes was simply existentialism. This conclusion is entirely natural from a Western point of view.
Existentialism is the rotting carcass left after you murder mysticism.
It’s a foul philosophy that denies a higher realm of existence, denies the sure conviction of a God in Heaven, and denies that morality is anything more than a sentimental construct. Existentialists understand that the world is absurd and that you have only your internal processes to build on, but they reject the ultimate source of those internal processes. They deny the heart or the heart-led way.
Just for the record, agnosticism is a tacit rejection of God. To deny that we can know God is merely the rejection of the heart as a sensory organ with its own mind. We do know; God has planted that in us. It is burned in more surely than any of us can claim to be alive. To deny the Creator, even to doubt Him, is to deny your own existence.
This is why we never debate or defend having faith. We should be ready to discuss what our faith demands of us, but you are under no obligation to answer anyone who a priori rejects the grounds of faith. In my experience, roughly 90-95% of the people you encounter in the US will insist on sticking with a Western epistemology or appeals to emotion (including American Christians). That leaves some 5-10% ready to discuss genuine faith. If you are able to explain that your epistemology is Ancient Near Eastern, you might get another 5% or so to take you seriously. However, you have no moral obligation at all to meet anyone on the grounds of Western assumptions.
Don’t debate agnostics and atheists. It’s a waste of time. In that sense, we never take them seriously. Everything they say is tainted by their closure of the heart. Until God opens their hearts to hear the truth, you can not do anything for them. Worse, they will be driven to destroy your faith. State your position when it seems appropriate; be on your guard, and keep them at arm’s length. Show civility and compassion when they have need, but that goes without saying.