You need to differentiate between an assumption and a premise or proposition.
Assumption =
a thing that is accepted as true or as certain to happen, without proof.
Google Dictionary.
Premise =
an idea or theory on which a statement or action is based
dictionary.cambridge.org/dictio … sh/premise
Note Scientific theories are qualified to some basic assumptions.
The point is these assumptions are agreed without questions by those who agree with Science, because these assumptions are reasonable.
It is the same with Mathematical axioms where all mathematical minded people will agree with them and others believe them based on faith.
In the OP case,
if there is a disagreement with the assumption, then the whole argument will not follow.
In this case, one will have to convince the assumption ‘God exists’ is reasonable.
Theists will agree with this assumption, rather they believe it based on faith.
But why should I accept this assumption.
If I cannot agree with the assumption then the following arguments is a ‘no go.’