I've answered this question many times throughout this thread, but I know how hard it is to actually read a long one before posting, so once again:
I'm mostly talking about ALL gods and
not any specific one, IOW, what would (
any god) need to do to prove (to you personally) that she was the real deal?
My definition of god is that she doesn't give a rat's a*** what bible or religion you follow as long as you follow the Golden Rule.
I've also answered multiple times why I don't think god is a male (or even a female really)., and the answer to why I think god is a she is, "Why not?"
I actually think if she exist, it's like what
Julian of Norwich felt in her
Revelations of Divine Love, and that god has both the spirit of a man and women in them.
Again it would work for absolutely everyone - god needs to do no more than rearrange a few synapses in your brain and you will believe it exists, that works for everyone.
Sorry, Darat, but a better answer (that wouldn't require anyone to lose their free will) would be to kill everyone and put them in front of the gates of heaven and then begin judging them.
ETA: That would also bring up another posit brought up earlier. Which religion would everyone be forced to believe in, Christianity, Islam, Judaism, Buddhism, Hinduism, or maybe something as simple as just...
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Well then, in that case it's like I said, in my post back when.
If you're not going to define God yourself, and leave people to define God as they like: well then, there's folks there who actually think God is all of nature, or that God is all of the universe.
(For instance, there's this guy:
https://steve-patterson.com/understanding-god-as-nature-or-the-universe/
...And I've seen this sort of thing in New Age books as well, I mean specifically where God is defined as the Universe, or as nature.)
So, if you're working with that kind of a definition of God personally, when then God doesn't have to
do anything at all. It already exists, evidently so. You already believe. Everybody already believes, given that definition.
So that, like I was saying, way back upthread: The simplest way to do what you're looking for is what Darat's suggested. Provided you don't insist on the not fiddling with free will caveat. ...And an equally straightforward way to do what you're asking for, is to simply define God in such a way that its existence becomes an incontestable thing. And this doesn't even require any diddling with people's free will*.
*Well, working with the religious idea of free will. I personally don't think free will exists, but that's completely irrelevant to this exchange and off-topic for this thread.
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eta:
What I've suggested ties in with the idea of igtheism or ignosticism. Essentially, God is a meaningless word, because it's such an open idea. Not just in terms of philosophical wanking, but actually --- provided one is able to move one's reference point beyond one's narrow tribe. If you look around the world, at different cultures, different God-ideas, then you'll see a huge diversity in what the term "God" even means. So that, to even ask the question, "Do you believe in God?", or, as you do, "What can God do to make you believe in Him", is a completely meaningless exercise, unless you first clearly define what exactly you mean by God. When you don't explicitly define God, then nine times out of ten, you're working with some implicit definition of God --- just as, in this thread, most people are implicitly working with the Judeo-Christian idea of God. Bring in pagan gods, and you'll need different parameters to make people believe in that pagan god. And bring in God-as-existence, or God-as-nature, or God-as-the-Universe, and you'll need to do nothing at all to make people believe in God.