it depends on what the best possible universe means.
It is simple, because God is all good, everything he creates is good and has no evil in it. He brings all things to himself. But what can be done in error is to look at the beginning of creation and say THIS must be perfect, obviously it is not so God must not exist.
To put it logically
God is all good and all powerful
The universe must be all good and have nothing bad in it.
clearly there are things that are bad in the universe
God isn’t all good and all powerful.
The logic of your first post doesn’t really work. All it proves is that God isn’t all good, not that he doesn’t exist.
But there is still a problem with this logic.
First those who argue this way don’t understand final causality.
For example, the fullness of an acorn is realized in it becoming a tree. Even though when it is planted in the ground it hasn’t reached it’s fullness, there is nothing wrong or bad in it. Sure it isn’t a tree, but it is moving towards its end.
We can understand that creation is like a seed. God in a sense, planted a seed that sprouted and is now growing into a plant. That plant is still growing and eventually it will reach it’s potential and bloom a flower. Until that point the plant has not reached it’s end it has not reached it’s perfection.
God didn’t create a universe that is stagnant and perfect from the beginning. Rather he created a universe that has an end, when it reaches that end it will be perfect and all good. Until it reaches that end, it won’t be perfect because perfection is only realized in a things fulfillment, or it’s end.
So let me restructure the argument using this idea of final causality
All things that that have an end, are moving towards that end.
If God creates things with an end than that end will be perfection
It appears that the universe is moving towards an end.
If God created the universe than it is moving towards a perfect end.
All existing things need God to create it.
The universe is an existing thing
Therefore the universe is moving towards a perfect end.
I think that works.
I may have to clean it up later.