"Rather, the court concludes that Colorado’s commission acted with hostility toward Phillips’ religious beliefs, as demonstrated by the commissioners’ anti-religion comments and refusal to sanction bakers who refused to sell cakes celebrating the traditional meaning of marriage."
The problem with this argument is that the bakers who were not sanctioned can't be said to have discriminated against gays or any other minority class. Therefore the different treatment is rational.
The court may still go this way, it's not always rational. But it would make no sense.
The real issue here is the free speech one, not the religious one. No person should be compelled to make speech they disagree with. A craftsman literally baking a message into a cake certainly applies. A makeup artist, although an "artist", is not making specific speech by their work. Even the baker offered to make a cake that didn't contain the message. This is actually much more straightforward than perhaps the justices saw. But maybe they did and just wanted to interrogate the lawyers.
Refusing to bake a cake upholding the traditional definition of marriage while agreeing to bake a cake supporting gay marriage would be discrimination based on religious belief, unless it could be proved that the reason to create such a cake was not a religious one.