Under Title VII, employees who have religious reasons for not masking are to be given reasonable accommodations.
See: https://www.eeoc.gov/wysk/what-you-should-know-about-covid-19-and-ada-rehabilitation-act-and-other-eeo-laws"

Particularly, look at this provision:

G.2. An employer requires returning workers to wear personal protective gear and engage in infection control practices. Some employees ask for accommodations due to a need for modified protective gear. Must an employer grant these requests?

An employer may require employees to wear protective gear (for example, masks and gloves) and observe infection control practices (for example, regular hand washing and social distancing protocols).

However, where an employee with a disability needs a related reasonable accommodation under the ADA (e.g., non-latex gloves, modified face masks for interpreters or others who communicate with an employee who uses lip reading, or gowns designed for individuals who use wheelchairs), or a religious accommodation under Title VII (such as modified equipment due to religious garb), the employer should discuss the request and provide the modification or an alternative if feasible and not an undue hardship on the operation of the employer's business under the ADA or Title VII.

U.S. Equal Employment Opportunity Commission

DOWNLOADS

Here are some helpful downloads to learn more about mask exemptions.

What You Should Know About COVID-19 and the ADA, Rehabilitation Act, and Other EEO Laws

Temporary Restraing Order - Douglas County

Federal Judge in Denver orders masks in all Douglas County, Colorado Public Schools to protect disabled students.  This order will likely trigger litigation and new public health orders all across the nation forcing all schools and all places of public accommodation to require masks or be violation of the ADA.

FREQUENTLY ASKED QUESTIONS

My employer or school requires masks. I have a religious objection. Do I have any recourse?

First, you must ensure that you have a bona fide religious objection. Few, if any, religions have specific teachings against wearing masks. You must be able to articulate a religious belief that the mask requirement violates. Medical, cultural, or political objections do not qualify as a bona fide religious objection. Some plaintiffs have sued based on religious objections to wearing masks in public, but courts have rejected those to date.

This information is from the Alliance Defending Freedom website as accessed on September 22, 2021.