An airman stationed at Creech Air Force Base in Indian Springs, Nevada, was prohibited from re-enlisting in the U.S. military last month for omitting the words “so help me God” from a service oath he was required to recite and for refusing to sign the oath containing the same words on his enlistment form, according to the American Humanist Association (AHA).

In a letter of complaint sent to the Air Force’s inspector general on Tuesday, the Appignani Humanist Legal Center, the AHA’s legal wing, said the airman — who is an atheist — “was told that his options were to say ‘so help me God’ or to leave the Air Force.’”

The AHA, which describes itself as “advocating values and equality for humanists, atheists and freethinkers,” characterized the ultimatum as a civil rights violation and demanded the Air Force correct the matter.

“Requiring [redacted] to take an oath containing this religious affirmation violates his clearly established constitutional rights under the First Amendment. This letter demands that you immediately allow [redacted] to re-enlist using a secular affirmation,” the letter stated.

“The Air Force cannot compel anyone to swear to God as a condition of enlistment,” AHA attorney Monica Miller said. “Doing so violates the establishment clause and the free exercise clause of the First Amendment.”

“Numerous cases affirm that atheists have the right to omit theistic language from enlistment or re-enlistment contracts,” she added.

In its letter to the Air Force, the AHA threatened that “commanding officers may be sued in federal court” if the airman is barred from reciting a “secular oath” to re-enlist.

However, in a statement sent to Al Jazeera on Friday, Air Force spokeswoman Rose Richeson said that the airman”s “term of service expires in November 2014” and that “he has until this time to complete” Department of Defense Form 4, which contains the oath with the words “so help me God.”

Another spokeswoman, Air Force Capt. Brooke Brzozowske, said Friday that taking oath was a mandatory for all service members. “Reciting ‘so help me God’ in the re-enlistment and commissioning oaths is a statutory requirement,” she said.

That was not always the case. Until October 2013, Air Force Instruction 36-2606, under which the enlistment oath falls, stipulated that “airmen may omit the words ‘so help me God,’ if desired for personal reasons,” The Air Force Times reported.

On Oct. 30, 2013, the Air Force appears to have quietly done away with that option. “The relevant section of that AFI now only lists the active-duty oath of enlistment, without giving airmen any option to choose not to swear an oath to a deity,” the Air Force Times said.

Officials at the Air Force public affairs office contacted by Al Jazeera were unclear why the rule had changed, saying they were gathering information regarding the circumstances surrounding it.

Citing the Air Force, The Air Force Times reported that the option to omit the words “so help me God” could not be reconstituted without congressional approval.