Okay, this is long and drawn out, but I think it's worth it to touch on some of the major parts of this whole issue.
During The McCarthy Era (it was an "Era"? Whodathunkit?) the President glomed onto the whole "under God" thing at the same time as McCarthy (mostly coincidental, BTW) in response to a percieved threat from the Russians after WWII that the US was not capable of responding to militarily. Specifically, I'm talking about the beginning of the Cold War. It was a way to differentiate Americans from Communists in the minds of the American public. It was pretty effective, too.
The issue here is, to make someone a "threat" in people's minds, you need to paint them first as "different" before you can make them "Enemy". After all, 99% of Russian Communists were/are caucasian, so there is no obvious difference between Russians and Americans, no easy way to paint a "Us" and "Them" picture, except that they speak a different language, but that doesn't apply because Spain, France and a whole bunch of other of our Friends (caps intentional) don't speak English. Worse, the Russians had been our Allies in WWI (an alliance of convenience, granted, but the average American didn't understand this), and took horrible losses supporting the cause every man, woman and child in America sacrificed so must to achieve, so in the minds of most Americans, they were our friends. Remember, most people at this time were not familiar with the phrase "the enemy of my enemy is my friend", they were just friends. Period.
Keep in mind, to most people different=strange=alien and from alien, it's a really small leap to enemy, and "they're godless, we're not" was a very quick, cheap and easy way to make this transition. The fast-food approach, if you will.
At the same time, the Soviets (during The Khrushchev Era) were quickly becoming a threat with the detonation of their first nuclear weapon in 1953. The Knights Of Columbus quickly responded to this new (and real) threat by beginning a campaign to add the words "Under God" to the Pledge. After all, it was illegal under the Communist Regime to practice any faith at all. They were godless. We, on the other hand, could practice any religion we wanted (as long as it recognized "God", in the singular apparently. I guess worshiping Zeus, Hera, Aphrodite and Apollo wouldn't have gone over well.).
So, when did "Under God" finally get added? 1954, one year after the USSR detonated it's first nuclear weapon. Not a coincidence. The USA had a new enemy, and the Cold War had begun.
Adding "Under God" to the Pledge was part of the mechanism which made accepting our former "friends" as "enemies", because "They" were godless, "We" were not, even though a good percentage of the US didn't subscribe to any organized faith. Keep in mind, it wasn't until Kennedy that we had a Catholic President, and we still haven't had a Jewish President. Even then, it was a big deal (no, huge) to have a Catholic as President. Not because Catholicism (per se) was so alien or unacceptable to the average American, but because the Roman Catholic faith was the publicly acknowledged impetus for some of the most horrific events in human history (Think: The Inquisitions, The Crusades, The Holocaust (yes, the Vatican endorsed it), Burning of "witches" and other heretics, do I really need to go on?), and the threat of that kind of thing scared many Americans. This was, after all, one of the major reasons we came over the pond in the first place, right? To get away from non-representative, Church driven government.
Of course, the USSR dissolved in 1991 under Gorbachev and the Cold War ended. With it, Russians were once again allowed to follow their faith, and faith as a whole was no longer verboten. Indeed, most Russians today practice a faith of some kind.
To summarize, we added "under God" to the pledge in order to assist our Government in painting the Russians as our enemy. I think it was a VERY Christian thing to do, personally. Make your own judgement about whether that's a good thing or not.
So, what is my opinion about "under God" in the Pledge? I think it should go. It served it's purpose, and no longer has a function. A select portion of our population will die trying to keep it there, or if it gets dropped, reinstated. The cynical side of me (se previous post) says that this is in essence brainwashing. The organized brainwashing of our children, mandated by the government. It implies that if you don't believe in a God, then you're un-American, when nothing could be farther from the truth. It's exclusionary (see the references to polytheism above), and it is just Anti-American. I'm against it being there officially. If you want to add it on your own, go right ahead. Just like when you take an oath in court, you have the option of NOT swearing on a bible. That option is there because if you swear to God, and you don't believe in God, without an alternative option, you can't be held accountable.
IF you want to read more about this history I've talked about here, and I recommend you do if you're interested, follow the links below:
Short history of the USSR (pops)
History of the Pledge of Allegiance (pops)
Persecution of the Jews, Support of the Holocaust by the Vatican
Burning of witches and heretics
The 4 (yes, FOUR) Inquisitions
The 7 major Crusades