A priori deductive reasoning is reasoning true before any observations are made. Such reasoning starts from basic definition and deduces from it by law of identity and law of equation (A = B, B = C, A = C).
P1. Divine being which comes into existence out of nothing will come into existence out of nothing even if such divine being didnt exist before.
P2. If such divine being will come into existence out of nothing, such being exists
C. Divine being exists.
Now, there is not much to disagree with any of these premises. P1 is just basic law of identity. Coming into existence out of nothing means "nothing --> something". P2 is likewise just basic law of identity. And C logically follows.
Divine being which comes into existence out of nothing is possible, as there is no any logical obstacle to it. But another problem for atheists is that any obstacle put in place to prevent "nothing --> something" can likewise have some possible being capable of overcoming that obstacle.