Why is religious freedom generally important?

The Constitution can only govern a moral and religious people.

"John Adams... wisely observed that: 'we have no government armed with power capable of contending with human passions unbridled by morality and religion. Avarice, ambition, revenge, or gallantry, would break the strongest cords of our Constitution as a whale goes through a net. Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other.'" 

Dallin H. Oaks, "Preserving Religious Freedom," speech given at Chapman University School of Law on February 4, 2011 (Part I) (citing Charles Francis Adams, The Works of John Adams, Second President of the United States, 228–29 (Books for Libraries Press, 1969). See also Elder Jeffrey R. Holland, "Faith, Family, and Religious Freedom," Clark Memorandum, Fall 2013, p. 25.

Religious freedom encourages higher moral standards than what the government requires under law, resulting in fewer violations and greater accountability.

Religions advocate for high moral standards and encourage accountability for wrongdoing. Religions also teach values such as service, gratitude, humility, honesty, and industry.
"Many scholars have gathered empirical evidence tracing the strong correlation between contemporary religious observance in America and virtuous behavior. For example, religiously observant citizens tend to be more generous and civically-minded neighbors... According to estimates, more than 90 percent of those who attend weekly worship services donate to charity, and nearly 70 percent volunteer for charitable causes." 

"How Religion is Vital to Society," Mormon Newsroom, Commentary, March 12, 2012, http://www.mormonnewsroom.org/article/religions-vital-place-society#_edn3.

The moral advancement of society is driven by religious principles.

"Many of the most significant moral advances in Western society have been motivated by religious principles and persuaded to official adoption by pulpit preaching. Examples include the abolition of the slave trade in England and the Emancipation Proclamation in this country. The same is true of the Civil Rights movement of the last half-century. These great advances were not motivated and moved by secular ethics or persons who believed in moral relativism. They were driven primarily by persons who had a clear religious vision of what was morally right."

 Elder Dallin H. Oaks, "Strengthening the Free Exercise of Religion," Part I of Speech given at The Becket Fund for Religious Liberty Canterbury Medal Dinner in New York City, May 16, 2013.

2. Religious freedom contributes to the ethical and moral advancement of society

Religious freedom allows individuals to act according to conscience.

"If we remove the rights of man to dictate according to their conscience, we also take away their responsibility for their own actions, and we bring their wrongdoings upon the controlling governments." 

Hans Noot, "Enhancing Freedom of Religion or Belief in the EU," European Interreglious Forum for Religious Freedom, October 20, 2013, http://www.eifrf-articles.org/Enhancing-Freedom-of-Religion-or-Belief-in-the-EU_a71.html.

"Can there not be a government in which majorities do not virtually decide right and wrong, but conscience? - in which majorities decide only those questions to which the rule of expediency is applicable? Must the citizen ever for a moment, or in the least degree, resign his conscience to the legislator? Why has every man a conscience, then? I think that we should be men first, and subjects afterward. It is not desirable to cultivate a respect for the law, so much as for the right. The only obligation which I have a right to assume is to do at any time what I think right." 

Henry David Thoreau, Civil Disobedience, http://thoreau.eserver.org/civil1.html.