Which why skin colour isn’t usually a barrier to joining a political or religious organisation.
But we’re not talking about “joining a political or religious organisation” are we, we’re talking about applying for a job at an outdoor activities centre. I’m reasonably confident that there are many workplaces still where the colour of your skin would absolutely prejudice the recruitment process.
This is why protected characteristics are protected. I’ve read comments on this very forum previously along the lines of “their face wouldn’t fit” as justification for rejecting interviewees (though not specifically referring to race). Which is understandable to a point, but if that ‘fit’ means you’re not employing Mohammed because the existing shop floor is predominantly Daily Express readers then the problem is the culture not the candidate.
In the case being discussed here, there is a difference between “you must adhere to our Christian values” and “you must be a Christian.” The former is perfectly fine, the latter would require justification in order to be lawful (AIUI).