Nope. No more that the uneven distribution of the understanding of calculus around the world proves that calculus is untrue. Something does not have to be universally believed to be universally true.
Calculus is much more accepted that Christianity though.
Schaefer is sort-of equating disbelief in Christianity with disbelief in calculus though. In other words, people would believe in Christ if only they knew about him and understood him properly. Unfortunately, every major religion makes the same claim.
This is not a problem that would vex an intellectual - it is a tenet of Christianity that belief is a matter of free will. The uneven distribution just shows that not everyone chooses the same way. No biggie.