Religious freedom advocates are appealing the U.S. military's decision to prohibit the sale of religiously themed dog tags, a move the military made after receiving complaints from a secularist group.

First Liberty, a religious freedom advocacy group, submitted a letter to the Army on Tuesday, asking it to lift a ban on the sale of dog tags that feature U.S. military logos alongside Bible verses. The group argued that the ban targeting dog tags sold by Shields of Strength, a "faith-based" business, violates the First Amendment's Establishment Clause, which prohibits the government from favoring or disfavoring a particular religion.

"What Shields of Strength is doing is perfectly permissible under the Constitution and the law," Michael Berry, director of military affairs at First Liberty, told the Washington Free Beacon. "It is the Army that's in the wrong here and they're the ones that need to take corrective action."

Shields of Strength has sold more than four million dog tags featuring trademarked U.S. military logos and Bible verses, many of them to service members. The company—which licensed logos from the Army, Marines, and Air Force—estimates that at least 9 out of 10 operational units have received the dog tags since 2002, according to the appeal filed by First Liberty.

The controversy started when the Military Religious Freedom Foundation, a secularist group, requested the military to prohibit Shields of Strength from selling the religiously themed products that featured military logos. The group wrote that the dog tags imply an "undeniable and incontrovertible endorsement of the Christian religion" in violation of Defense Department regulations and the First Amendment.

"This is a licensing matter. You can't take the logo of the U.S. military … to benefit yourself," MRFF president Michael Weinstein said. "Imagine if … Liberty University wanted to buy the Army logo and put it all over their stationaries. You can't do that! Our constitutions don't allow it."

In August, the Army, Marines, and Air Force complied with the MRFF demand, prohibiting Shields of Strength from selling the dog tags featuring trademarked logos. The Department of Defense did not respond to requests for comment.

MRFF said in a press release that the company told the Marine Corps that it will stop selling the dog tags, but Shields of Strength's website still sells Army and Air Force-themed dog tags that cite Scripture. The company also continues to sell other dog tags that do not contain Bible quotes. Company officials did not respond to requests for comments.

First Liberty intervened on behalf of Shields of Strength, submitting its petition to the Army. The group, which also plans to take action against the Marines and Air Force, is willing to use "all legal options" to compel the Army to reinstate Shields of Strength's license to produce the dog tags, according to Berry.