One can absolutely know for sure, that He exist. Whether one believes in Him or not, saying the following will take nothing from your day. “I sincerely call on The One Who Created me, if You exist, make it clear to me, set my heart firm on it & protect me from all faleshoods that will keep me away from You. If You don’t exist & I am not one worthy of Your Mercy & Guidance, then leave me as I am. And let the cards fall where they may for me.”
That’s it. Then just go on with your life, either you will believe in Him or you will stay the way you are.
But just know, that just because one doesn’t believe in a thing, doesn’t make it not true. A blind person can’t see that the sky is blue, but others are telling him it is, but he chooses not to believe them, but it doesn’t change the fact that the sky is blue.
That same blind man is walking down the street & many are warning him, some gently, others screaming, there’s a hole the size of 30 city blocks & if you keep on your path, your gonna fall in & never come out. But he chooses not to believe the kind words of advice nor the strong, straight to the point words of advice, to stop going on that path, but it didn’t change the fact that the hole was there, & he willing walked & fell in.