An unnamed airman in the United States Air Force wants to continue to serve his country. Yet, the Air Force reportedly told him that his service is unwanted unless he swears an oath that concludes with the religious affirmation “so help me God.” According to the Air Force Times, the airman crossed out the words “so help me God” when he signed his reenlistment contract. He was subsequently told that he must either swear this religious oath or leave the service.
Think about that for a moment. The Air Force is asking someone to take an oath of loyalty to their country. Now that's the key part: they need to ensure that the person enlisting is genuinely committed to "support[ting] and defend[ing] the Constitution of the United States against all enemies, foreign and domestic". They need to know that that person will "obey the orders of the President of the United States and the orders of the officers appointed over" him or her. But none of that has to do with a god.
Furthermore, it's in the Air Force's (and the country's) best interests to maintain the meaning of that oath, and the commitment with which it's said. But by forcing someone to pledge to a belief that they might not hold they are actually compelling a patriotically person to either 1) forgo serving our country or 2) pledge to something they don't actually believe. Neither of these are beneficial outcomes. We have a volunteer force, so we want committed volunteers. And we want that oath to be as sincerely meant as it can be. Introducing extraneous elements, and making service contingent on them, is not beneficial to anyone involved.
...Congress did pass a law stating that members of the armed forces should swear an oath that includes the words “So help me God,” [but] the Constitution trumps an act of Congress, and requiring servicemembers to comply with this portion of the law is almost certainly unconstitutional. In the 1961 case Torcaso v. Watkins, the Supreme Court held that “neither a State nor the Federal Government can constitutionally force a person ‘to profess a belief or disbelief in any religion.’”
Admittedly, the courts often show greater deference to the government in military matters, but the Supreme Court has also indicated that this deference does not permit servicemembers to be forced to swear a religious oath. “The test oath is abhorrent to our tradition,” the Court stated in its 1946 decision Girouard v. United States. “Over the years, Congress has meticulously respected that tradition and even in time of war has sought to accommodate the military requirements to the religious scruples of the individual.”