This is an interesting, complex question. It would seem it could be well argued a private business, exercising their religious conscience, is protected in the Constitution, and that, to try and force anybody to violate the scripture of any established religion they believe is a violation of their Constitutional right of free exercise. Likewise, to try and force free exercise on somebody's private premise that doesn't want to be, in some way, actually involved in that exercise would be a violation of their right to not have religion forced upon them. For instance, I think it's a violation to force children to pray in school, despite my being a Christian, if we are to consider U.S. law. It must be emphasized that what does or does not legally stand is not a matter of the Bible governing, rather the laws of the nation. You could believe, in all good conscience, at least your own misguided conscience, that the U.S. should be a theocracy, but you would have no legal standing in this. (The question is always whose theology, expecting corrupt and narcissistic man could possibly run a theocracy.) Would further state God is not interested in forced prayers, that are not in a spirit of truth, those of you that somehow think the Lord Jesus has ever wanted any forced homage from the unrepentant, at least in this age of grace. It would also be degenerate hypocrisy for the U.S. government to persecute Christians, given the Western style nation largely came about, in its inception, due to Christians seeking to escape the persecutions in Europe. Did you know it was Christians who wanted no established religion, not atheists, not Satanists, this to avoid the persecutions of Europe? The atheists or Satanists would like to repudiate or deny this Christian heritage, but there would, in fact, be no Bill of Rights, if it were not for Rhode Island and Virginia Baptists demanding no government mucking in religion or restricting free exercise of Christianity, before they would endorse the Constitution. This is historic fact, and you may as well suck it up. Like it or lump it, there was a Christian design process at work in the founding of the United States. It's another matter if the U.S. has been Christian, and it's not my intent to go there, anyway, in this matter of cakes.
Of course, this would only apply to a private premise, not a government entity, which cannot be involved in establishing any religion. In other words, if the government baked cakes, it would have to bake Christian and Satanist cakes, because the government is not allowed to establish religious edicts, though it would be questionable whether the government would be allowed to create cakes with religious statements, in the first place. As precedents stand, I believe a government bakery would be precluded from putting any religious message on a cake, of any kind, would have to deny all religious cakes. Ironic how the government, that ruled against the Christian baker in the homo case, would turn down, discriminate against, people who wanted Christian cakes, if it had a cake shop? Just a weird thought. Bottom line, though, people need to realize that it's American law that will govern in this instance, not the Bible, and, throughout history, the conscience and faith of people has been known to be challenged by governments. But I believe a Christian baker's religious free exercise, civil rights, are violated, to be forced to do anything against their conscience, also a Muslim baker, if they refused to bake a Christian cake, and that it's up to the unregenerate public to find a baker that doesn't care, or make their own devil cake, then. To say anything different is to make a ludicrous claim that religious faith has no validity outside of church walls, when it's simply a fact that exercise of faith, in good conscience, is a total lifestyle, that moral precepts of religious faith, obviously, go with the person, not the confines of any building.
So, I believe somebody coming into a private premise, dwelling or business, and trying to force sin upon them as their scripture dictates, should have to take a hike, find a sinful business person to cater to their sin. That would be the next ten bakers they could go to, as things stand, but I see it as a clear violation of free exercise to force anybody to bake a cake of sin, or in any way punish them for refusing. In other words, this is a case of violation of Constitutionally guaranteed right of free exercise, not a theological debate matter, Christian theology having nothing to do with it, where the law is concerned, a law which is to respect the exercise of faith in good conscience, period, that the Constitution and free exercise govern in this case.
To say it’s discrimination for a private entity to have any exclusive rules, then businesses should have no dress codes, no speech codes, no conduct codes, whatsoever, in matters protected: if bakers can’t refuse a Satan cake, then office buildings shouldn’t be allowed to exclude a host of legally protected behaviors, lest they discriminate. Do you know there are people who believe smoking marijuana is a religious rite? I say, if you can’t exclude a Satan cake, you can’t exclude smoking pot in the break room, since the precedent would be everybody should have free exercise, in anybody else’s house. If you can’t refuse that Satan cake, a lot of law needs to be overturned, lest we discriminate. It would also be the case Christian churches should be forced to read from the Quran, maybe sacrifice some cakes to the Queen of Heaven, play heavy metal music, since nothing is excluded in the law on a private premise, right, lest we discriminate? Muslims mosques forced to read from John 1? For all I know, nudity may be a religion to some, hence a need to overturn laws mandating clothing at bakeries, isn’t that so? Shouldn’t that occult book store be forced to sell Bibles, also? Metallica forced to sing Amazing Grace?