I think it is by considering that the actual problem isn't the dichotomy Mahayana vs. Theravada or other such religious dichotomies. But that instead, the core problem is what the purpose of religion as such is. Is religion something that should make it possible for a person to live a "good life" according to worldly standards? Is it to transcend the world? Is it something else altogether?
It's not clear how such a thing can be a matter of personal decision. From the perspective of the mainstream Western religiological discourse it certainly seems like a personal decision. But this discourse assumes a number of problematic assumptions about the epistemology of religious choice (e.g. assuming people have chosen their religious beliefs while it cannot be meaningfulyl said that they did), while its other assumptions are mutually exclusive with the tenets of some religions (e.g. one cannot choose karmic determinism, as karmic determinism precludes personal choice).This is ultimately a personal manner and there is no single solution. Ultimately everyone has to decide if they think that they will hold the Mahayana sutras and shastras as definitive.
How could one possibly decide which discourse about reality is right one, the true one? If this is to be up to one's own choice, then the person's religiosity comes down to simply affirming their own beliefs.