God exists is a perfectly reasonable logical statement.
What you are doing is asserting a truth for the purposes of making logical deductions. For example, you could take this statement, add some others then apply logical rules to deduce further statements.
Now, whether the logical system you have produced is of any practical use or interest is a separate matter. Even if it's completely meaningless it can be, nonetheless, a perfectly valid logical system.
All dogs are yellow, yellow things quack therefore dogs quack is a perfectly valid, perfectly useless logical scheme.
That's not to say making logical statements that are controversial is inherently pointless. For example, say you incorporated this statement with others. Now, you apply logical operators and ended up with the statement
God does not exist. You now have a logical inconsistency which would imply, in your logical scheme, that God's existence is a logical contradiction. Whether this ends up as being known as Hamid's Paradox or whatever will, of course, depend on exactly how uncontroversial the other statements turned out to be.
It's also probably worth pointing out that, as God's existence is an input to your logical scheme, the only valuable thing regarding God's existence that you could reasonably deduce would be the contradiction above.