Religion plays a vital and important role in many people's lives, and of which, many a country has historically had a favored or expressed authorized religion that they exclusively support, in which, those that kowtow to such, are the beneficiaries so of, and those that do not, are prone to suffer the consequences of having an unorthodox belief. In the case of England, that country purposefully broke away from the Roman Catholic Church in the 16th century, to establish their own religion, thereafter known as the Church of England, of which, the “Defender of the Faith” of that religion was the same personage as the Monarch of England, thereby signifying that the population answered to that Monarch, in their faith as well as in their everyday life.
A significant number of countries do not desire to have a population that would be susceptible to having a divided loyalty, especially when that loyalty is divided between this world and the afterworld, of which, the workaround for precluding this, is typically to have a governmental sponsored orthodox religion that all must pay obeisance to. In England at the time of their Religious Act of Uniformity, all those that failed to adhere to the terms of that uniformity, by, for instance, not faithfully worshiping at the authorized church each and every week, were subject to monetary fines; and if caught in the reading of unauthorized religious devotionals or through their physical attendance at a state banned religious institution could thereby have initiated against them criminal sanctions.
The United States, though, was founded upon a different and a highly noble principle, which they made part and parcel of their national creed, which is that freedom of worship, deserved to be a Constitutional right, freely given to all, and that therefore it so followed that the national government could not ever have an established religion of their own, which that government continues to faithfully honor to the present day. So then, whether a given person is religious in the traditional sense or not, freedom of worship always matters because this essentially represents whether or not the common citizen is susceptible to having their freedom of conscience, violated, by the power and intrusion of the state.
Any government that has stolen a given person's sovereignty of their conscience, has taken away the most critical element of what it so means to be at liberty, for when that government tries to control, to monitor, or to manipulate the minds of its population, it has forsaken being a government so created for the general welfare of the people, to instead representing outright oppression by its entering into the very minds of its people, for often its own nefarious purposes.
When it comes to our religious faith, our beliefs, our thoughts, and our dreams, these are ours to own, and it is not the business of any legitimate government to interfere in such, but rather it is their business to see that these are properly protected and remain inviolable. All those that would attempt to control us, don't seem to rightly comprehend that free will, and free beliefs, are what define us as human beings, for we are not meant to be automatons.