You don't have to convince me that Smith and Mohammad were crappy people. However, that's not my point. Both of those men are far better attested to than Jesus, but the fact that they existed does not convince Christians that their more extravagant claims are true. "This person existed" is simply not sufficient.
On the other hand, there are millions --billions, even-- of people who do believe that Smith and Mohammad were divinely inspired. Why? Presuppositions. Very powerful, yet very clearly unreliable. If we apply the same approach to all of these claims, they all fall short. To the extent that they are believed, it is entirely dependent on what we believed before we had a chance to put them to the test.