Yes you do in fact know that God exists! You believe in God no matter what words come out of your mouth. Now you are saying to yourself “I don’t believe you!” Well Why then do you not act as you profess to believe?
If you say that you do not believe in God, then love is only electro-chemical reaction in the brain. Why then do you kiss your wife when you leave for work? Maybe you are even ok with being an adulterer, but there is something in this world that you believe is objectively morally wrong (hopefully murder).
In order for there to anything objectively morally wrong there must be a non-arbitrary source of that morality. Human thinking is arbitrary so that cannot be it. Nature is constantly changing within set boundaries so that can’t be the moral source. Morality is only applicable to conscious beings and are conceptual so Morality must be the product of a sentient mind. There is only one thing that can be called a non-arbitrary sentient mind and that is God.
Let’s say that you still don’t believe in morality. Well you actually can prove the non-existence of something. There are two methods of doing this:
1) through contradiction
See if something does not exist, then it is not logically possible. Such as a square circle or a married bachelor. These are contradictory to each other therefor they do not exist. But morality is not only logically possible but must exist for logic to exist. You see, there must exist moral value in order for you to have a reason to identify anything. Therefore the first law of logic is contingent on moral value.
2) Cause and effect
When something does exist, there is an observable and even measurable (by time) cause and effect from that object or entity. If a person says that they are wearing an invisible hat, I could either prove that the hat exists or not through the cause and effects. Such as when they wear the hat, is there a ring around their hair line? Is the temperature under the hat hotter while the person wears the hat? And so on! See Cause and effect!
Since you you are refrained or restricted in your actions, this is evidence of the effect of morality. An effected that adhere to even if you do not declare a belief in it with your statements.