One way I approach the question is this: are the ‘stories’ of the god(s)-in-question the ‘same’ stories?
In general, the story of the God worshiped by Christians is like this:
Creation→Fall→Israel→Jesus Christ→The Church. I think those are the necessary ‘plot points’ that all Christians could agree on. Any other story would not be ‘the same’.
It is not a very sophisticated approach, but I like it for its usefulness: everyone knows what a story is, after all, and comparing stories, comparing plot points, brings into sharp relief whether we are talking about the ‘same’ Author.
A quick follow-up to my post re: ‘stories’ above.
If I was discussing the question of the ‘same’ God with a Muslim (as in fact I have), I could give the short story as follows, avoiding metaphysics to a great extent:
“The God I’m talking about created the universe and all that is in it including mankind; and pronounced it all ‘very good’.
The representative Man, Adam, chose to sin and therefore gave up his high calling, and as a result of this Fall, sin infected the human race like a disease, which explains both the glory and the misery of humans. This Fall also affected the creation itself, according to St. Paul.
The call of Abraham was the beginning of God’s healing the creation/mankind. Thus eventually Israel came to be, and was given the privilege of bringing God’s love to the world. Apparently, they failed. It’s right there in the scriptures.
God sent His Son Jesus in the fulness of time, the last Adam, Who accomplished in Himself what Adam and Israel did not. Those who are joined to Jesus in truth and in spirit become the Church, whose high calling and responsibility is to live and love their neighbors, and spread a new but good ‘disease’ - the love and spirit of God.
The Church has the first four acts of a five-act play -(I got this from Tom Wright): Creation, Fall, Israel, Christ - and is called upon to divine the Author’s intent and methods, and act out the final act.(To be followed by a much greater Story, so we’re led to believe).”
That is scandalously abbreviated of course - but I could ask my Muslim friend - is that the story of Allah? Is The Father of Jesus Christ, as understood by Christians, the SAME as Allah?
That’s why I think the story approach can be useful.
So I’m thinking that the question is not the degree of devotion a disciple has, or the fervency with which he/she for the tenets of the faith, or their sincerity - when we are asking ‘Who is Right’, we are inquiring into the cogency of, the comprehensiveness of, the degree of correlation between, particular stories and Reality. As for ‘knowing’ we are right - that’s a biggie. I go along with GMAC’s father who told him “All a man can do is to choose to believe”; and belief has many roots, intellectual among them.