What basis do you have that the religious tenet is newly invented? Even if it were, what's the basis to uphold that something newly invented is not a sincerely held belief?

OP is clearly posing a hypothetical. And courts absolutely look to the amount of time and frequency with which one practices a tenet of their religion as evidence of sincerity. OP might be sincere, but a court would be unlikely to use that as evidence to justify, for example, a wrongful termination claim.

The Supreme Court just needs to decide it's not sincerely held.