Phrases like "thinking exclusively in terms of probabilities" don't get us anywhere unless you are talking to someone who say 'Yes, I believe that not-P is more probable than P, but I believe P anyway, as a matter of faith." Now a thoroughgoing fideist might say something like that, but someone who is that much of a fideist would probably not bother to argue with you. Who you are likely to encounter here are people who think the evidence for their religious beliefs shows their beliefs to be more probable than its contradictory. The idea of faith, to them is simply trusting the one whom they think they have good reason to believe in.
Faith in God is trusting God, and so I don't see any real problem with the concept of faith as commonly used by Christians. The fact that they use such a concept in no way implies that they are closet fideists or anything like that. Someone could have faith in a spouse in a very different epistemic situation, a situation in which the evidence that the spouse is having an affair is very strong, but the person persists in having faith in their spouse nonetheless. Even here there are two scenarios. One of them is where the person says "Yes, the evidence suggests that she's having an affair, but I choose not to believe it." The second is where the person says that the evidence supports their spouse's fidelity. In the first case, you have cause, perhaps, to complain about the "leap of faith" they might be taking. In the second, the person is not taking a leap of faith, they are just misassessing the evidence and the probabilities.
If someone thinks that Christianity is probably true, then it's not going to be much of an issue if you tell them to think in terms of probabilities. If someone is believing that Christianity is true even though their best reasoning tells then the weight of the evidence is against it, then of course you might try to say they shouldn't have faith. If your assessment of the evidence is correct, then "reasonable faith" would not be instantiated anywhere, at least where faith concerns beings like God. But "reasonable faith" would not be an oxymoron, a contradictory concept. The world could have been such that reasonable faith is instantiated. But, on your view, it just doesn't happen to be that way.