Yes, there is great interest in religion in America, and yet government largely entertains that interest without defining the word 'religion.'
If you go here, you find the on line searchable US Code.
Type in the word 'religion'
The word 'religion' appears many times in the US code, but at best, they skirt around a definition. And although you might think that the IRS 'must' define religion, they in fact do not, because they cannot.
Yet...the US COde is filled with definitions for commonly understood words such as 'person' and 'employer' with the form "the term person shall mean..." in a Definitions section, explaining the use of that word in the context of the law at hand.
So there are belabored definitions such as :
Sec. 2000e. Definitions
For the purposes of this subchapter--
(a) The term ``person'' includes one or more individuals, governments, governmental agencies, political subdivisions, labor unions, partnerships, associations, corporations, legal representatives, mutual companies, joint-stock companies, trusts, unincorporated organizations, trustees, trustees in cases under title 11, or receivers.
(b) The term ``employer'' means a person engaged in an industry affecting commerce who has fifteen or more employees for each working day in each of twenty or more calendar weeks in the current or preceding calendar year, and any agent of such a person, but such term does not include (1) the United States, a corporation wholly owned by the Government of the United States, an Indian tribe, or any department or agency of the District of Columbia subject by statute to procedures of the competitive service (as defined in section 2102 of title 5), or (2) a bona fide private membership club (other than a labor organization)
... and so on.
So, search for the literal "term religion" ... and you get exactly two hits.
One is as follows:
For purposes of this section--
(1) the term ``Indian'' means a member of an Indian tribe;
(2) the term ``Indian tribe'' means any tribe, band, nation, pueblo, or other organized group or community of Indians, including any Alaska Native village (as defined in, or established pursuant to, the Alaska Native Claims Settlement Act (43 U.S.C. 1601 etseq.)), which is recognized as eligible for the special programs and services provided by the United States to Indians because of their status as Indians;
(3) the term ``Indian religion'' means any religion--
(A) which is practiced by Indians, and
(B) the origin and interpretation of which is from within a traditional Indian culture or community; and
(4) the term ``State'' means any State of the United States, and any political subdivision thereof.
... without defining 'religion'...
...and the second hit is the following, which is of no help at all in defining a narrow definition of 'religion:'
(j) The term ``religion'' includes all aspects of religious observance and practice, as well as belief, unless an employer demonstrates that he is unable to reasonably accommodate to an employee's or prospective employee's religious observance or practice without undue hardship on the conduct of the employer's business.
That is the raw material that the USSC interprets-- the Law of the Land. That is the topic of consideration for every USSC case in existence. It is the body of legislative work generated under the license of the constitution.
And...it includes no narrow definition of the term 'religion.' Not one. "all aspects of religious observance and practioce, as well as belief" can hardly be defined as a 'narrow' definition of the term 'religion', or even, a non-circular definition.
Defining 'religion' as 'religious' is akin to defining the word society as "an enduring and cooperating social group whose members have developed organized patterns of relationships through interaction with one another" and then defining the word "social" as "of or relating to human society, the interaction of the individual and the group, or the welfare of human beings as members of society "
Self-referential mystery upon mystery, world without end, indeed.
But the bottom line is, especially if you read the intent expressed in the US Code, a bias towards broad tolerance of expressed religion in this nation, as opposed to a narrow definition. It is exactly the foundation of our freedom.
An example from the US Code:
From the U.S. Code Online via GPO Access
[Laws in effect as of January 3, 2007]
TITLE 22--FOREIGN RELATIONS AND INTERCOURSE
CHAPTER 73--INTERNATIONAL RELIGIOUS FREEDOM
Sec. 6401. Findings; policy
Congress makes the following findings:
(1) The right to freedom of religion undergirds the very origin and existence of the United States. Many of our Nation's founders fled religious persecution abroad, cherishing in their hearts and minds the ideal of religious freedom. They established in law, as a fundamental right and as a pillar of our Nation, the right to freedom of religion. From its birth to this day, the United States has prized this legacy of religious freedom and honored this heritage by standing for religious freedom and offering refuge to those suffering religious persecution.
It shall be the policy of the United States, as follows:
(1) To condemn violations of religious freedom, and to promote, and to assist other governments in the promotion of, the fundamental right to freedom of religion.
(2) To seek to channel United States security and development assistance to governments other than those found to be engaged in gross violations of the right to freedom of religion, as set forth in the Foreign Assistance Act of 1961 [22 U.S.C. 2151 et seq.], in the International Financial Institutions Act of 1977, and in other formulations of United States human rights policy.
(3) To be vigorous and flexible, reflecting both the unwavering commitment of the United States to religious freedom and the desire of the United States for the most effective and principled response, in light of the range of violations of religious freedom by a variety of persecuting regimes, and the status of the relations of the United States with different nations.
(4) To work with foreign governments that affirm and protect religious freedom, in order to develop multilateral documents and initiatives to combat violations of religious freedom and promote the right to religious freedom abroad.
(5) Standing for liberty and standing with the persecuted, to use and implement appropriate tools in the United States foreign policy apparatus, including diplomatic, political, commercial, charitable, educational, and cultural channels, to promote respect for religious freedom by all governments and peoples.