The courts really do not want to get into the bussiness of dictating people's religion. They explicitly do not require that a religious belief be well supported by the text, orthodoxy, or any denomination of the religion. Nor do they require that a religious belief is self consistent or unchanging through time. It is possible to argue that a belief is not sincerly held, but it is very difficult to win such an arguement. As such these legal arguemnts are almost always about what constitutes a 'reasonable accomodation'.

Perpetual employment is not a reasonable accomodation; but if you work in HR at a large company, you could probably get out of handling the firing of other employees.

In practice what this means is that if an employee says they have a strong personal conviction about something, the employer must offer reasonable accomodations if any are available, or be prepared to show that the claim was made in bad faith.