You can know for certain that God exists because there is no other way for the physical universe to come from physically nothing.
“Nothing comes from nothing.” (How could it, there’s nothing there to produce something.)
Also, there is no other reasonable explanation for design, complexity, and purpose other than God.
People say, “chance,” created the universe, but chance has no creative or organizing or life-giving or purposive power.
Consciousness, conscience, and self-awareness are not possible without God.
Also, there is no other (ultimate) explanation for the Moral Law other than God.
We have all broken the Moral Law. This is known as, “sin.”
God, being holy, Who created the Moral Law, must punish sin, or He would not be holy and righteous.
The only way out of these absolute problems of absolute reality is God’s gracious forgiveness of sins through our believing in Jesus, the Son of God, God Incarnate, true God and true man, crucified for all of your sins, dead, buried–and risen on the third day.