The purpose of a definition is to clarify what is being discussed.
The definition I am giving you of God is "The Ultimate Reality" which is in line with what the church teaches. Besides that, it is the definition you will find in Merriam-webster's collegiate dictionary. The Oxford uses "Supreme Being" which means the same thing.
Your entire argument is to say that God is something other than this. Thus, you aren't talking ahout the same thing but you act as you are .
You have a non-argument.
And it isn't about me being true. It is about God being true, because God is The Truth. You cannot make an argument that The Truth is a lie. That there is no Truth. If you say, "There is no truth", you undermine your own assertion.
Of course you know this, because you admit that reality exists. That is why you must make God something other than God to have an argument. Otherwise, your argument would amount to, "Prove to me that it is true that there is truth! Ha! You can't! I win!".
And even this whole appeal to epistemological nihilism is a cover. It isn't that you don't know, it is that you know better(in your own reasoning). It is something that comes from pride, even though to the undiscerning it might look like humility. "Oh, he doesn't know". But you aren't simply saying "I don't know." You are saying that nobody can know(because you know better). You are also asserting that God doesn't exist.
So do you know God doesn't exist? Not at all. You don't even know what God is. You are uneducated, and because you are an unteachable know-it-all who can't stand the idea of not being the smartest most well informed person in the room, you in your pride must knock everyone down to your level. You fancy yourself scientific. But I tell you, if you were in. a room of engineers, they would laugh at you for trying to impress them with your nonsense, and tell you to take your know nothing bullshit to the dirty pot smoking drum circle kids by the river, who might actually be impressed if they weren't as arrogant as you are.