When our founding fathers drafted the First Amendment to the Constitution stating "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof," they likely viewed religious diversity as variations of the Judeo-Christian theme prevalent throughout Europe and the colonies at the time. But things are much different today.
So, the first question we should be discussing is: Do we really mean "any" religion? Satanism is on the rise in countries like Poland, and it has become so prevalent that its practice is now allowed in the British Royal Navy.
What if the Aztec religion, complete with horrific human sacrifice and its policy of conversion or death, suddenly enjoyed resurgence? What if some religion worshipped a god of fertility and practiced pedophilia?
Do we truly believe Americans should be allowed to practice any religion? If not, we are abandoning a founding tenet of American liberty. And where do we draw the line?
I'm guessing the majority of Americans have limits to what they believe is acceptable as religion. But that's not what we espouse. We need to resolve this dichotomy.
First, "human sacrifice" and child molestation are not allowed under U.S. law, period, not even in the name of religion. Dan Linssen is grossly ignorant of constitutional law if he really thinks these are unsettled questions. He needs to look at, for example, the Supreme Court decision in Church of the Lukumi Babalu Aye v. City of Hialeah (1993), which ruled on the legality animal sacrifice.
The 1993 Supreme Court ruling held that no law can single out animal sacrifice. But animal sacrifice can still be prosecuted under other, more general laws, e.g. against cruelty to animals, depending on the locale. Thus, animal sacrifice - at least of some species of animals - might be legal in many rural areas, where hunting is allowed and where people are allowed to slaughter their own farm animals, but still illegal in most urban areas. The point is that animal sacrifice must be subject to the same laws that govern animal-killing in general, whatever those laws might be. (See my page about animal sacrifice.)
Laws against murder and child molestation are general. They do not single out any religion. Thus, no religion is allowed to commit these crimes as part of its religious practice.
Be that as it may, the worship of "fertility gods" need not entail "human sacrifice" or child molestation. Nor does it typically involve such activities, at least in today's world. Ditto for Satanism. See the Church of Azazel statement against violent crime and vandalism, for example. There are plenty of law-abiding Pagans, and there are plenty of law-abiding Satanists too.
By the way, the decision by the British Navy to allow a Satanist to practice his religion does not imply that Satanism has become "prevalent" there.
Anyhow, Pagans and Satanists should take note of the following: See how Dan Linssen appears to be using the current wave of anti-Muslim scaremongering to argue that the government should consider taking away our rights too. And indeed, if even the constitutional rights of Muslims can be abridged, then all the more so can the rights of smaller religious minorities. We must stand up for across-the-board religious freedom, limited only by generally applicable laws with a secular purpose.
(Dan Linssen himself seems educable, though. On his blog, he does not come across like a full-blown religious right wing demagogue. However, it is highly likely that plenty of right wing demagogues will be using arguments similar to his in the not-too-distant future.)