Well they must not have believed as strongly as Christians do seeing how those religions were regional and short-lived while Christianity has been a widespread and very longlasting religion (including Judaism as its precursor).
Well, it is fairly hard to maintain your religion when an army of Christian missionaries decend upon you killing everyone who disagrees with them.
Additionally, are we suggesting that whichever religion has been around the longest is most correct? Or whichever religion has the most followers?
Is "truth" that relative?