The argument is somewhat circular.
You start out with the premise "Nobody has ever believed X while X wasn't true".
But you also now, presumably, have someone who believes X. And you're using that to prove X is true...
...but if X ISN'T true, then the premise "Nobody has ever believed X while X wasn't true" is also not true.
In other words, you have to assume X is true in order to maintain the claim "Nobody has ever believed X while X wasn't true", which you then go on to use to support the claim that X is true.