I think the greatest miracle is when a person discovers and embraces the truth. I wonder if any miracle can be proven beyond all doubt.
Even for those of us who do believe in God, there’s always a tiny seed of doubt that must be (as I see it) challenged. Doubt is not a deal-breaker, but if we use it to make ourselves ascertain that we are on the right track, it’s a good thing, because we are not merely following blindly.
An atheist who doesn’t want to know will not hear or agree with any arguments for the existence of God. I think the only really beneficial thing for them is prayer in the hope of my aforementioned greatest miracle.