"gullible" is too strong a word and you're right varian faith cannot be proved any more than it can disproved to the one who has that faith.
As for "it appears to be the prerogative of the faithful to easily take offence if anyone questions their faith", I disagree. Questioning someone's faith isn't the same as insulting someone for the faith they hold and/or insulting what that faith is.
I am not intolerant of those who question my faith, but I am intolerant of those who insult my faith which is extremely important to me, and/or insult me for having that faith.
What has survived in your garden with minimal or no watering?

