I believe that "not religious" is considered a "religious" belief. I'm pretty sure that "religious" beliefs can even include things like not wanting to take a vaccine (according to some courts).

You can. That’s the point.

You can’t, because this court has also decided that religious beliefs deserve more protection. The lack of religion means you have less power in the USA.

You absolutely can refuse to take on a job designing Christian content on the basis that it conflicts with your religious beliefs.

There are "public accommodation" laws that involve this, but they are pretty much saying that "public accommodation" doesn't extend to creative endeavors, like making websites or baking custom cakes.

The headline is wrong and this is a free speech case, not a religious case, specifically the question of "whether applying a public-accommodation law to compel an artist to speak or stay silent violates the free speech clause of the First Amendment."

The idea is that the government can't compel someone to endorse a particular message. This includes the commission of creative works that someone disagrees with. This is NOT a general right to refuse service based on protected characteristics.

So you can't make a Jewish baker bake holocaust cakes if they don't want to (yes, this is an actual example that came up). But if someone is already selling holocaust cakes, they have to sell them to everyone regardless of race or sexuality.