With the arguments for the existence of god or indeed gods so weak, the theologian retreats to the idea that it is all about faith, that is, a leap of faith to believe in something that cannot be arrived at by reason. By this means they can argue that their belief in god is not something that can be argued about rationally and hence religion is a conversation at which science or rationality does not have a voice. Completely coincidently of course, this gets them off the hook for, in the case of christianity, not being able to come up with a half decent proof despite two thousand years to do so. There are two very obvious problems with this supposed killer "proof" that nail it dead.
If we accept for argument's sake that the leap of faith is a valid argument, then we run into an insurmountable problem. Having by definition excluded reason from the process we are now left in the position of leaping off the cliffs of rationality into one of the multitude of boats below, each a mutually contradictory religion, but aside from the knowledge that one of them is correct, with no way of choosing the right one! We cannot use reason to decide the merits of each before choosing (this is a leap of faith remember), so we have a very limited choice: choose none or choose randomly. If this is a search for truth, choosing none is the only valid choice. This also illustrates to my mind, that if someone makes a leap of faith to the religion they already believe in, they are either deceiving themselves or us.
Secondly, everyday experience tells us that under ordinary circumstances the vast majority of people do not change the faith they were indoctrinated in as children. Indeed Islam goes as far as proclaiming the death penalty for its apostates to put them off doing so, while the Catholic Church recently changed its rules to make it effectively impossible to officially leave, as a result of the Irish Count Me Out campaign. If there was fluidity in people's religion, it might suggest that the proposed leap of faith is being made. The carrying over of childhood indoctrination into adulthood religiosity suggests that people assume their religion is true because it is their religion. There are a multitude of religions out there with conflicting claims and gods and all claiming to be true. Self-evidently at most only one of these can be true. There is clearly no active engagement on the part of most to genuinely seek out the 'true' religion, so no leap of faith either, as that implies more than impassive acceptance. Additionally if there was such engagement, it would be reasonable to assume some general migration towards a single religion.
As an aside to this point, of course there have been times when populations have changed their religion en masse (hence the caveat of ordinary circumstances above). This happened frequently in reformation Europe for example when the idea of the king's religion becomes the people's religion appeared. This reinforces the point though as these conversions are essentially political and nothing to do with the truth of religion.
As a closing point on religious proof, my experience and general impression is that outside of theologians, that is for the vast majority of believers, there is in fact little engagement with how to prove god exists or that a particular religion is true, and that if more people were given, and took the time to understand, an honest review of the evidence for god, they would be shocked at the flimsiness of the arguments.