If all reality is within my mind, then all reality is representative (intentional).
The intentional relates to something other than itself.
Therefore, though any particular intentional thing might relate to another intentional one, all of them cannot (one can’t have a whole made of all relations, whether they are arranged in a circle or go on forever).
So there is some reality outside my mind. Call this intrinsic as opposed to intentional reality.
Whatever is in the mind is relative.
the relative depends for its existence on the non-relative.
Therefore, what is in the mind depends on something outside of it.
Let God be the one whose intrinsic reality is greater than mine.*
If God himself were not the cause of my idea of him, then a being with less intrinsic reality than God suffices to explain my idea of him.
But then an idea would have a relation to more than what sufficed to explain its existence,** which is impossible.
Therefore, God exists.
*If there is more than one such thing, the greatest of them.
**There is nothing wrong with an idea having more logical implications than what gave rise to it (like the equality of persons) but we’re not considering logical implications in this argument.