I have had relatively few religious debates where I did not hear my counterpart assert that “I don’t need proof, I have faith”.
The first, and funniest, thing to note about this claim is how defensive it is; arriving only after your counterpart realizes that what they thought could pass for proof really can’t. Only then will they pull out what they believe is an ace in the hole – hilariously claiming they need no proof after they have tried and failed to produce some.
At this point, simply explain to your counterpart that faith is no more than a personal opinion. As soon as this sobering point is understood, you will be able to circle back to the objective/subjective debate, which can easily be dismissed. A simple courtroom analogy with the defense pointing out that the prosecution has no evidence for their case, and the prosecution claiming that it needs no evidence because it has faith, would clarify how silly this argument is. If silly arguments like this would get laughed out of any respectable courtroom, why would anyone think they could establish infinitely larger and more consequential arguments like those put forth by religion?
Or why not simply tell your counterpart that you too have faith (i.e. – a personal opinion), and that your faith is based on evidence. Then ask your counterpart what their faith/personal opinion is based on.
When the answer to this question comes – usually in the form of vague pseudo philosophical babble about known unknowables or knowing through “other” means – kindly explain that when it comes to objective truth claims about the nature of the universe, one’s level of certainty should be proportioned by the amount of evidence in support of this claim. In other words, lack of evidence does not, in and of itself, mean you are wrong, but it does mean that your claim is baseless (at least until evidence is produced), and that you should perhaps preface your claims with “I’m not exactly sure of this but…” or “I don’t have any evidence for this, but it’s my personal opinion that…” And yet, most of what you’ll hear from the faithful is how absolutely, unequivocally, literally and unshakably sure they are of their unsupported thin-aired faith based assumptions.
It will also be useful to remind your religious counterpart that if faith alone could establish an argument, then anyone’s faith could establish the truth of any argument – from the promise of the 72 heavenly virgins, to lord Xenu populating the earth with extraterrestrial souls, to the existence of Santa Claus – all would have to be considered real if we actually based objective truth claims on whether or not people have faith in them. Speaking of which, let it not be overlooked that the faith of an Islamic suicide murderer, who cheerfully obliterates himself in a crowd of children or flies a jetliner into a building, must be quite a bit stronger than that of, say, an average evangelical Christian preacher or an orthodox Jewish rabbi. Let us, therefore, not stoop down to any games of arithmetics by trying to calculate truth through degrees of faith.
Finally, and I do not know why I’m putting this last, if faith alone can establish the truth of an argument, then not only can a different faith replace it, but a lack of faith can refute and dismiss it altogether. One’s faith in something like transubstantiation, or heaven, or Joseph Smith’s babblings about Jesus having visited upstate New York, are in no way more real or powerful than my lack of faith in these claims.
If you can will a thing into existence, I can will it out of existence; if your can construct an argument out of faith, then I can deconstruct it through a lack of faith.
What can be asserted without evidence can also be dismissed without evidence.