Rescher's Ontological Argument

1. The concept of God can only be had in virtue of a body of experience which is sufficient to justify belief that God exists. (Premise)
2. I have the concept of God. (Premise)
3. Hence, I am justified in believing that there is at least one God.

How to argue for 1?

4. A concept is essentially experientially based if it can be gained only by means of a body of experience. (Definition)
5. Any body of experience sufficient to gain an essentially experientially based concept is sufficient to justify belief that something falls under that concept. (Premise)
6. The concept of God is essentially experientially based. (Premise)

But is 6 true? Maybe the concept of God is only essentially partly experientially based. Let's try a variant:

7. A concept is essentially partly experientially based if it can be gained only by means of a body of experience combined with other concept-acquiring processes. (Definition)
8. Any body of experience that, possible together with other concept-acquiring processes, is sufficient to gain an essentially partly experientially based concept is sufficient to justify belief that something falls under that concept. (Premise)
9. The concept of God is essentially partly experientially based. (Premise)

But 8 is false. The concept of a red square circle is a counterexample.

So, I don't know if Rescher's argument can be defended.