This is something I was first intrigued by late, after a nationwide billboard campaign a couple of years ago that had a fake quote from George Washington: I think "It is impossible to rightly govern the world without God and the Bible" was what they had plastered across the country. This quote was invented in the 1860s based on an 1835 biography of Washington that said something similar--and not even from anything Washington wrote, but as a decades-later remembrance from an unidentified source.
I started thinking about this again because I've noticed a lot of Founding Father religious quotes--mostly fake--suddenly popping up on Facebook. Here's one I've already seen a few times, a bogus bit of James Madison invented in the 1930s:
“We have staked the whole future of American civilization, not upon the power of government, far from it. We have staked the future of all our political institutions upon the capacity of mankind for self-government; upon the capacity of each and all of us to government ourselves, to control ourselves, to sustain ourselves according to the Ten Commandments of God."
Another from Patrick Henry, one of the few things that great orator didn't say:
“It cannot be emphasized too strongly or too often that this great nation was founded, not by religionists, but by Christians; not on religions, but on the Gospel of Jesus Christ."
There's also this tidbit supposedly passed by Congress in 1782...
"The Congress of the United States recommends and approves the Holy Bible for use in all schools."
...which is also a complete fabrication. At that time the Federal government really didn't have much to do with schools at all, certainly not "approving" anything for them--and in fact public schools as we know them didn't exist yet.
So where do these quotes come from? A lot of the ones circulating on the Internet today actually found their source in one modern book: David Barton's 1989 The Myth of Separation, in which he attacks the separation of church and state using quotes that are either phony, paraphrased (usually badly), or taken out of context. And by "phony" I mean that some of the quotes don't appear anywhere before his book.
The thing is, it's easy to pass off a phony quote. Just write something that sounds formal and you can probably get away with calling it a passage from the Founders. To wit, take this phony James Madison anti-religion quote from, let's say, "1788", shortly after the adoption of the Constitution:
"Dear sir, you ask me about my resolve in placing my judgment as such, concerning a lack of conviction about a Supreme Being within our new constitution. I can only state that provision for the acknowledgment of the Deity was not made in our compact due to the overriding conviction that as we threw off the tyranny of one secular monarch, so too did I feel compelled to abrogate the worship of another such tyranny either in the form given by the Old Testament, or the ancien regime of the worst of old Europe's religious excesses."
So where did that quote come from? I made it up, just now, after skimming the style of a few of Madison's letters.
See, wasn't that easy? Nearly anyone can do it. And a lot of people have.
What I don't quite understand is why people feel the need to make up phony Founder quotes when there are legitimate ones they could use--Washington's 1796 Farewell Speech has plenty of nuggets that could be considered pro-Christian, for instance.
Maybe people like Barton think the quotes aren't overt enough--that they could even be subject to multiple interpretations. So--like early monks or later translators who altered the Bible's text to suit the weather of the times--they decided to create what they think the Founders should have said, or what they "surely" meant.
So how do you recognize a fake quote? Well, chances are if it's...overt, for lack of a better word...it's not real. If it doesn't have an attribution beyond the speaker, at least on the Internet, then chances are it's not real. But an attribution doesn't guarantee authenticity. It's not unheard of for a fake source to be given, and sometimes that source doesn't even have anything to do with religion. The pseudo-quote "Do not let anyone claim to be a true American if they ever attempt to remove religion from politics" is often wrongly credited to Washington's Farewell, for example.
There are also more specific tags (or anti-tags). George Washington, for example, never wrote the name "Jesus Christ" at all in any of his writings but one, his 1779 address to the Delaware Indian chiefs. (And even that is often misquoted.) If you see him mention the full name anywhere else, it's fake.
Sometimes there are also anachronisms, like a Washington quote where he supposedly referred to "prairie wagons". Which didn't exist until after Washington died.
Just keep a skeptical heads up and chances are you won't be led astray.