They are not Messengers *of God.* They are Messengers who are sent by God. They are evidence that God exists since they are sent *by God.*
But only if you can demonstrate that they are sent by God. And to do that
requires that you *first* show that a God exists.
No, it is not possible to *first* show that God exists since the only way to know that God exists is the Messengers that are sent by God.
And that is precisely why the whole argument is circular. You need to know that God exists to know the messengers are sent by God, and you need to know they are sent by God to have evidence that God exists.
It does not matter if it is circular because that does not mean it is not true.
But it does mean that there is no evidence. To be evidence requires lack of circularity.
Circular reasoning is a logical fallacy in which the reasoner begins with what they are trying to end with.[1] The components of a circular argument are often logically valid because if the premises are true, the conclusion must be true.
Circular reasoning - Wikipedia
So...
If the premise Baha’u’llah was a Messenger of God is true, then the conclusion God exists must be true.
Yes, but you need a way to show the premises are true that doesn't involve the circular argument. At some point, the circle needs to be cut.
And if the premise that Baha’u’llah was a Messenger of God is false, then the conclusion that God exists need not be true. So you need some *independent* way to show that Baha’u’llah was a Messenger of God that does not require the previous assumption that there is a God.
Notice that circular reasoning *is* a logical fallacy. The only way to break it is to *independently* prove one of the premises in the circle.