Way back in the mists of time, on May 19th, 1991, I received my commission in the United States Navy. Before doing so, I had to take an oath of office. That oath went as follows:
I, [name], do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God.
Origin. The current oath has remained unchanged since 1884. The simple and direct core of the oath - to support the Constitution of the United States - actually dates from much earlier, having been established in 1789. As such, the oath dates from the earliest days of the United States.
Purpose. The purpose of the oath is to support and defend the Constitution of the United States. Note that nothing is said of the Congress, the President, the States, or any other individual, group, or organization. As far as the oath is concerned, the Constitution - the primary law of the land - is the only thing that matters.
Allegiance. While the oath is administered by an individual, it is never sworn to that individual. In fact, it is not sworn to any individual, group, or institution. The oath is is one of obedience to the rule of law - the Constitution itself.
Duration. Indeterminate. As the Wikipedia article on the United States Uniformed Oath of Office explains, "The oath is for an indeterminate period; no duration is specifically defined." In other words, this is not an oath to be taken in haste, or renounced lightly. It is an open-ended commitment that is specifically not linked to the duration of time served in any particular office.
There are about 1.5 million men and women serving in the United States military. There are an order of magnitude more veterans - about 22 million, according to the VA. That's roughly 10% of the population who have at one point in their life solemnly sworn that they would be willing to lay down their life, if needed, in order to preserve the Constitution.
As Leonidas said, "Μολὼν λαβέ". A phrase that I think applies whether you are speaking of my guns, my speech, or my worship; or my Constitution.
And I do not think that I am alone.