My minimalist explanation is:
The words "under God" were added to distinguish America from the godless, communist Soviet Union.
The Soviet Union fell 25 years ago.
Communism has died in some countries and is dying in the others.
With the Soviet Union deceased and communism dying, what remains of the purpose of those two words?
There's more than that to persuade atheists and other non-theists, including the little-known motives of a few Americans in the early years of the terrible 1930s Depression. They feared Americans would lose faith in capitalism.
You are very correct, Tom. They keep the pledge that way because they are pandering to christians. They want that vote. Even the modern christians do not know the truth of what you wrote. I solve the question by avoiding the pledge altogether. I might stand up when others do, etc. but I avoid "hand over the heart" or any other crap. I had enough of it years ago. For those who say I'm not "patriotic" I ask you what we would do in Brazil, Africa, or Germany?
I rest my case.
The words under God were added at Eisenhower's suggestion in the 1950's. The pledge I learned as a child did not have them. I simply pause for two beats.
The British author Llewelyn Powys was appalled at the pledge when he first heard it at a time when the salute resembled the Nazi salute (for which reason it was changed).
One afternoon during that winter, I attended a lecture that my brother gave at a certain women's club. Before he spoke, the two hundred and fifty members who made up his audience all stood up and began to recite, like a set of Sunday-school children, a patriotic hymn, and at a given moment thrust out long arms in the direction of the American flag, I was, I must confess it, a trifle taken aback. Here was a gathering of women who were probably the leaders of society in this particular neighbourhood, and yet they apparently felt no misgiving at taking part in so provincial a display. When I considered the intellectual aridity, the lack of taste, capable of producing such a ceremony upon such an occasion, I could only gasp. Yet the set expression on those female faces, as they pointed at the Stars and Stripes. just as savages might point at the totem of their tribe, has remained always in my memory, to remind me, when I grow unduly optimistic, of the unenlightenment which lies like a miasmic mist in the way of any charming and tolerant civilization.
Llewelyn Powys, The Verdict of Bridlegoose, Chapter IV