The First Amendment guarantees Americans the freedom of religion in the "establishment" clause:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Words matter, so the first question that must be answered is a matter of definition. What is religion?
The dictionary defines religion as:
1. The belief in and worship of a superhuman controlling power, especially a personal God or gods.
2. A particular system of faith and worship.
3. A pursuit or interest to which someone ascribes superhuman importance.
Dictionaries have been used for centuries to help codify the meaning of words in an attempt to make language useful. Without accepted specific meanings for words it is impossible to communicate through language effectively. Language is the common denominator of speech. Even biblical stories express the importance of the meaning of words as they are understood or misunderstood in any language. The most famous example is the biblical story of The Tower of Babel that begins with everyone on Earth speaking the same language and able to understand each other. Whether the scattering of people around the world was a punishment for hubris or not, the consequence was that people began speaking different languages and could no longer understand each other.
But what happens when people speaking the same language no longer understand each other because they interpret the meaning of the same words differently? That is the situation we are facing in contemporary American society today.
The second question that must be answered is a matter of interpretation. What does religion mean to you?
Thomas Jefferson wrote eloquently on the subject in an 1802 letter to the Danbury Baptists who worried about their minority status in Connecticut. Jefferson was reassuring the Baptists that being a minority religion would not be a problem in a Protestant majority state as far as the federal government was concerned.
"Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof', thus building a wall of separation between Church & State. Adhering to this expression of the supreme will of the nation in behalf of the rights of conscience, I shall see with sincere satisfaction the progress of those sentiments which tend to restore to man all his natural rights, convinced he has no natural right in opposition to his social duties."(Wikipedia)
Jefferson's letter clearly indicates that for Jefferson, religion was a matter of Man and God. Jefferson's interpretation was the widely accepted and understood view of religion in the early 18th century. By the 20th century the U.S. Supreme Court "incorporated" the Establishment Clause and expanded its application from the federal government to the state governments as well.
The practical application of the freedom of religion also requires a uniform understanding of the meaning and interpretation of the word religion. The Exercise Clause clarifies the supremacy of Constitutional laws and freedoms over religious laws and freedoms. This is particularly important in contemporary America because we are facing "religious" practices of Islam that threaten our Constitutional freedoms.
The Free Exercise Clause distinguishes between religions beliefs and religious practices. It is the equivalence of distinguishing between thinking and doing. In America an individual is free to think murderous thoughts but he is not free to murder. Islam is a religion governed by religious Sharia Law that endorses honor killings, female genital mutilation, murder of apostates, murder of homosexuals, wife beatings, child marriage and pedophilia. American jurisaprudence does not have the will or authority to change people's beliefs whether they are citizens of the United States, guests in this country, here illegally, or citizens of other countries, but we most certainly have the right and legal obligation to disallow any and all practices in conflict with the U.S. Constitution and our cultural norms. Free Exercise Clause (Wikipedia)
"Freedom of religion means freedom to hold an opinion or belief, but not to take action in violation of social duties or subversive to good order."[28] In Reynolds v. United States (1878), the Supreme Court found that while laws cannot interfere with religious belief and opinions, laws can be made to regulate some religious practices (e.g., human sacrifices, and the Hindu practice of suttee). The Court stated that to rule otherwise, "would be to make the professed doctrines of religious belief superior to the law of the land, and in effect permit every citizen to become a law unto himself. Government would exist only in name under such circumstances."[29] In Cantwell v. Connecticut (1940), the Court held that the Due Process Clause of the Fourteenth Amendment appliedWhile the right to have religious beliefs is absolute, the freedom to act on such beliefs is not absolute.
In Jefferson's time as in Truman's time the meaning of the word religion was understood as items 1 and 2:
1. The belief in and worship of a superhuman controlling power, especially a personal God or gods.
2. A particular system of faith and worship.
Seventy years later in 2017 we must reconsider the meaning of the word religion and ask the question What is Islam?
Islam is not a religion like Buddhism, Christianity, Hinduism, or Judaism. Islam is a unified supremacist socio-political system with a military wing and a religious wing. Islam is governed by religious sharia law. The goal of Islam since the 7th century is to make the world Islamic and impose sharia law worldwide.
Islam is tyrannical in its demand for conformity to its barbaric sharia laws. Islam is intolerant. Islam is a political force seeking world dominion and cannot be allowed religious protections like the Baptists in Connecticut during Jefferson's times.
Islam is far more like the Nazis during Hitler's time. Consider this question. What if Hitler declared Nazism to be a religion. It certainly qualifies as a religion according to Item 3. A pursuit or interest to which someone ascribes superhuman importance.
If Adolph Hitler declared his Nazism a religion would the left-wing liberal apologists for Islam defend Nazism and its determination to rule the world and rid the Earth of every Jew? Would the lefty-wing liberals declare murder of Jews protected by religious freedom? How is this different from allowing Muslims to perpetrate honor killings, female genital mutilation, murder of apostates, murder of homosexuals, wife beatings, child marriage, and pedophilia.
There is no difference.
If, as apologists for Islamic barbarity claim, Islamists have perverted their religion - then it is also true that they have perverted our concept of religious freedom. Islam is not a religion like any other and its savage practices do not deserve protection under our religious freedom laws and the free exercise clause.