Here is the key passage of the 1989 SCOTUS opinion (Employment Division v. Smith) that the Oregon court cited as the basis for ruling against the Kleins on the 1st Amendment argument. I've removed most of the case citations to make it more accessible. Smith was a split ruling and Congress responded to it by passing the Religious Freedom Restoration Act. The Oregon court didn't buy the Kleins' argument that the Oregon statute was passed to target the Christian religion.
The Free Exercise Clause of the First Amendment, which has been made applicable to the States by incorporation into the Fourteenth Amendment, provides that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof . . . ." The free exercise of religion means, first and foremost, the right to believe and profess whatever religious doctrine one desires. Thus, the First Amendment obviously excludes all "governmental regulation of religious beliefs as such." The government may not compel affirmation of religious belief, punish the expression of religious doctrines it believes to be false, impose special disabilities on the basis of religious views or religious status, or lend its power to one or the other side in controversies over religious authority or dogma.
But the "exercise of religion" often involves not only belief and profession but the performance of (or abstention from) physical acts: assembling with others for a worship service, participating in sacramental use of bread and wine, proselytizing, abstaining from certain foods or certain modes of transportation. It would be true, we think (though no case of ours has involved the point), that a State would be "prohibiting the free exercise [of religion]" if it sought to ban such acts or abstentions only when they are engaged in for religious reasons, or only because of the religious belief that they display. It would doubtless be unconstitutional, for example, to ban the casting of "statues that are to be used for worship purposes," or to prohibit bowing down before a golden calf.
Respondents in the present case, however, seek to carry the meaning of "prohibiting the free exercise [of religion]" one large step further. They contend that their religious motivation for using peyote places them beyond the reach of a criminal law that is not specifically directed at their religious practice, and that is concededly constitutional as applied to those who use the drugfor other reasons. They assert, in other words, that "prohibiting the free exercise [of religion]" includes requiring any individual to observe a generally applicable law that requires (or forbids) the performance of an act that his religious belief forbids (or requires). As a textual matter, we do not think the words must be given that meaning. It is no more necessary to regard the collection of a general tax, for example, as "prohibiting the free exercise [of religion]" by those citizens who believe support of organized government to be sinful, than it is to regard the same tax as "abridging the freedom . . . of the press" of those publishing companies that must pay the tax as a condition of staying in business. It is a permissible reading of the text, in the one case as in the other, to say that if prohibiting the exercise of religion (or burdening the activity of printing) is not the object of the tax but merely the incidental effect of a generally applicable and otherwise valid provision, the First Amendment has not been offended.
We have never held that an individual's religious beliefs excuse him from compliance with an otherwise valid law prohibiting conduct that the State is free to regulate. On the contrary, the record of more than a century of our free exercise jurisprudence contradicts that proposition. As described succinctly by Justice Frankfurter in Minersville School Dist. Bd. of Ed. v. Gobitis: "Conscientious scruples have not, in the course of the long struggle for religious toleration, relieved the individual from obedience to a general law not aimed at the promotion or restriction of religious beliefs. The mere possession of religious convictions which contradict the relevant concerns of a political society does not relieve the citizen from the discharge of political responsibilities." We first had occasion to assert that principle in Reynolds v. United States, where we rejected the claim that criminal laws against polygamy could not be constitutionally applied to those whose religion commanded the practice. "Laws," we said, "are made for the government of actions, and while they cannot interfere with mere religious belief and opinions, they may with practices. . . . Can a man excuse his practices to the contrary because of his religious belief? To permit this would be to make the professed doctrines of religious belief superior to the law of the land, and in effect to permit every citizen to become a law unto himself."
Subsequent decisions have consistently held that the right of free exercise does not relieve an individual of the obligation to comply with a "valid and neutral law of general applicability on the ground that the law proscribes (or prescribes) conduct that his religion prescribes (or proscribes)." United States v. Lee, 455 U.S. 252, 263, n. 3 (1982) (Stevens, J., concurring in judgment); see Minersville School Dist. Bd. of Ed. v. Gobitis, supra, at 595 (collecting cases). In Prince v. Massachusetts, we held that a mother could be prosecuted under the child labor laws for using her children to dispense literature in the streets, her religious motivation notwithstanding. We found no constitutional infirmity in "excluding [these children] from doing there what no other children may do." In Braunfeld v. Brown, we upheld Sunday-closing laws against the claim that they burdened the religious practices of persons whose religions compelled them to refrain from work on other days. In Gillette v. United States, we sustained the military Selective Service System against the claim that it violated free exercise by conscripting persons who opposed a particular war on religious grounds.
Our most recent decision involving a neutral, generally applicable regulatory law that compelled activity forbidden by an individual's religion was United States v. Lee. There, an Amish employer, on behalf of himself and his employees, sought exemption from collection and payment of Social Security taxes on the ground that the Amish faith prohibited participation in governmental support programs. We rejected the claim that an exemption was constitutionally required. There would be no way, we observed, to distinguish the Amish believer's objection to Social Security taxes from the religious objections that others might have to the collection or use of other taxes. "If, for example, a religious adherent believes war is a sin, and if a certain percentage of the federal budget can be identified as devoted to war-related activities, such individuals would have a similarly valid claim to be exempt from paying that percentage of the income tax. The tax system could not function if denominations were allowed to challenge the tax system because tax payments were spent in a manner that violates their religious belief."