For me, the existence of God is THE most important question in the universe. If there is no God & life is a cosmic accident, then nothing really matters. You're alive for a few years, then die. Eternity past & eternity future have no meaning. Your short time of existence is a blip on the radar screen. Eternity & the universe take no notice of you.
But if there is a God & an afterlife of sorts, then that seems to me to be the most important factor in our lives.. if what we do & think during this short time affects us for eternity, then the stakes are much higher. IF there is a spiritual realm, then finding out about it would seem to be a fairly important part of our world view, & would affect our plans, lifestyle, interaction with others, & overall outlook.
IF there is a God, then another thing can be deduced: God has shrouded himself in mystery, & has not plainly revealed himself to his creation. He seems to not be bothered by dogmatic assertions from the religious or irreligious alike, & lets us bicker, fight, & kill each other to defend or promote our ideas of what he is like. I cannot deduce what his purpose in that might be.
There is Truth. There is an ultimate Reality. Most (if not all) of us just do not know what it is. We speculate, deduce, & reason with what information we have, but we do not know.
Eccl. 8:14 There is futility which is done on the earth, that is, there are righteous men to whom it happens according to the deeds of the wicked. On the other hand, there are evil men to whom it happens according to the deeds of the righteous. I say that this too is futility. 15 So I commended pleasure, for there is nothing good for a man under the sun except to eat and to drink and to be merry, and this will stand by him in his toils throughout the days of his life which God has given him under the sun.
16 When I gave my heart to know wisdom and to see the task which has been done on the earth (even though one should never sleep day or night), 17 and I saw every work of God, I concluded that man cannot discover the work which has been done under the sun. Even though man should seek laboriously, he will not discover; and though the wise man should say, “I know,” he cannot discover.