The Medieval Era of Philosophy is theocentric. One of the famous discussions from this era was St. Anselm's ontological arguments.
I am Anselm of Canterbury, and I have arguments to prove that God exists.
St. Anselm argued that God, as the greatest being conceivable, must NECESSARILY EXIST. His arguments opened important discussions that led to the perspectives that we know today.
I am Gaunilo of Marmoutiers, and I would love to hear your arguments
God is that than which nothing greater can be conceived. Don't you agree?
Indeed. To exist is even better than be a mere thought. So, God, as the greatest thing imaginable, MUST necessarily exist.
Of course. God is a perfect in our minds. But to exist in the reality is far better than existing in the mind.
We can conceive of an idea of a perfect being by starting from the less perfect beings and ascending to greater attributes until we form an idea of a perfect being.
But as mere humans, we cannot think nor imagine an idea and reality like God. If we even dare to do so, it would only limit God's unlimited greatness.
But, still, imagining a perfect being does not necessarily prove its existence. I can think of a perfect island that has everything I need and want but it does not mean that it truly exists.
However, your perfect island is not the greatest thing conceivable, which is why it does not exist.
There cannot be more than one thing than which nothing greater can be conceived because having a co-equal being means that it is not the greatest being. Only God is the perfect being that cannot not exist.
As a Benedictine monk, I do agree that God is a perfect being. I merely cannot believe your explanation of His existence. Nonetheless, let's agree to disagree and continue improving our philosophies.