When secular laws are used to prevent the free exercise of religion and force behavior that goes against those religions, it is acting as a competing religion.
Again though, if you have to use the claim that this means that all secular laws are unconstitutional maybe it is your claim that is wrong and not the fact that secularism is acting as a competing religion...
If secularism is a religion, then its laws are religious, just as Sharia law is a body of law that regulates public and private life in Islamic states. I don't believe secularism can be described as a religion, nor do I think the founders thought so. I do believe secular law can conflict with the First Amendment freedom of religion, and as has been pointed out by others here, it has done so with respect to Mormons, Indian rituals, and religious tests for public office. Just as with the free speech provisions, some secular laws put limitations on it. When conflicts take place, for example in public education, it can get messy and require judicial remedies.
When secular laws are used to protect other rights or aspects of the Constitution, and those protections conflict with religious laws, which takes precedence? If it's religious law, then religious laws are superior to the constitution. John Adams views notwithstanding, I doubt the framers had that in mind, given they declared the Constitution the supreme law of the land.
My opinion is that laws that force creative effort by a business which clearly conflicts with religious principle are not constitutional. Laws that require goods and services to be provided to all not involving creative effort (ie: inventory) are probably constitutional. If I call a cab and a Muslim driver won't pick me up because I may be a Christian, do I have a cause of action. I hope so.
Just My very humble opinion, of course.
