No Religious Test Clause
Article VI, Section 3, also known as the No Religious Test Clause of the Constitution, is a small line of the text which stipulates that for applicants seeking to serve in the federal government, there can be no religious test barring their entry to office upon election or appointment.
The text reads: "but no religious test shall ever be required as a qualification to any office or public trust under the United States."
This however does not mean that religious tests cannot be applied elsewhere as a force of law. Immigration laws, for example, commonly employ religious tests.
Section 1158 of Title 8, U.S. Code states:
the applicant must establish that race, religion, nationality, membership in a particular social group, or political opinion was or will be at least one central reason for persecuting the applicant.
Additionally, Section 1101(a)(42)(A) of Title 8, U.S. Code specifically states what a "refugee" is and again, religion (a religious test) is stipulated in federal law.
The reason for this qualification in law is simple: If a refugee is claiming religious asylum on the basis of persecution, the claim of their belief and the claim of their persecutor both need to be established and proven.