“I consider it pretty doubtful that probability is a meaningful concepts for propositions not involving events which are (in principle) repeatable, such as the existence of God or of an infinite number of parallel worlds.”
That hearkens way back to A.J. Ayers and the logical positivism movement, which had its way on the philosophical scene in the 1940’s and 50’s. Ayers himself was of the opinion, based on his reading of Hume, and Wittgenstein’s Tractatus ( Ironically it was the latter’s Philosophical Investigations that helped put the nail in the coffin of the Positivists) that any proposition that could not be ‘verified’ by repeatability (i.e., “scientific method”) is nothing more than an assertion, a mental state. Similarly, to give a probability to a statement is not to say anything meaningful about the statement, but only to express the confidence (or lack of it) we have in it, e.g., a mental or emotional state.
But that of course is stacking the deck by determining the criterion for what is true, before any investigation is even made. It suffers from the fatal mistake that Wittgenstein drove home in the Philosophical Investigations and the theory of the language-games.
If you are by ‘decree’ laying down the parameters of the discussion and saying “This is the field we will play the game on, and these are the rules”, why then you’ve guaranteed yourself a ‘win’, because your opponent may not not be playing that game; his game may be played on a much larger field with much larger goals. And a much different Referee 
If verifiability in Ayer’s sense is the only way we can claim either truth or probability, we are left with a very reduced and ultimately meaningless world-view, in my opinion.
I think also that in regards to atheists : many of them fall into the trap of the verifiability game and its stress on reduction of reality to a very narrow compass of so-called ‘knowledge’. But that reduction of reality really cannot satisfy our human need for transcendence, for fulfillment in God.
In short, if we are playing the ‘prove it to me’ game with an atheist, the meaning of ‘proof’ has to be expanded far beyond the scientifically verifiable, into the realm of human meaning, and love, and the Great Dance (CSL). Then everything else starts to make sense in light of that.
I’m very happy you have joined the forum, Lothar’s son, and appreciate your comments though I disagree with them (at times).
Have you read, personally, the Philosophical Investigations?
