Just a friendly reminder that the argument’s first premise is not to be taken as a general modal principle that p -> p. (I wouldn’t blame anyone for balking if it were.)
Rather, it is stating that the subject of the (Tisthammer) proof, G, where G = G, will be found to exist necessarily if it is found to exist in actuality. But at this stage of the proof, it has not yet been determined that G qua G exists at all, either actually or necessarily. There is no proof before the first premise that necessary existence itself even exists, and that’s why the first premise is necessary and is found in every version.
The first premise is nothing more than a compaction or restatement of the conclusion from the proof of necessary existence itself, where p = p in any arbitrary world, and where p -> p is the S4-Axiom. In other words, G -> G is a reduction of G -> G. To quote Stanford, “In S4, a string of operators of the same kind can be replaced for that operator…”. And that applies to S5 as well, of course.
Throughout the argument, until the conclusion, you could substitute G for G, since G is what G represents. There are some versions that show Brouwer’s Axiom, p -> <>p, in order to make the notion more obvious. (B added to S4 yields S5.) But Brouwer is really redundant in this case because this version invokes Becker (modal status, except for actuality, is always necessary) in the third premise, which should make it fairly plain what is going on with premise 1. Usually, versions that don’t use Becker do use Brouwer, and vice-versa.
It’s possible that that’s why some people have said that the proof “defines God into existence”. It isn’t always made plain what is meant by saying that premise 1 follows from the definition. It follows from the definition because, and only because, G -> G reduces to G -> G when G = G either by Brouwer or by Becker.
Regarding DJ’s point, I agree with him. This proof says nothing about the nature of God beyond the mere nature of His existence, i.e., that it is necessary. It assigns to God, as existence, a predicate that is also existence of a particular kind. Any consideration about God being Jehovah, of the divinity of Jesus, or that sort of thing is entirely outside the scope of the proof.
All that can be taken from the proof is that IF He is sentient, then He is perfectly sentient. And so on. Sorry for the interruption. Please carry on.