Abraham Lincoln used it in his Gettysburg Address in November 1863, when he made a short (2 minutes) speech during the dedication of the War Cemetery. Won't quote the entire speech here (Google it if you wish) but I do recall that he had drafted the speech, using the words "one nation", then later inserted the words "under God" in the following passage:
"It is rather for us to be here dedicated to the great task remaining before us -- that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion -- that we here highly resolve that these dead shall not have died in vain -- that this nation, under God, shall have a new birth of freedom -- and that government of the people, by the people, for the people, shall not perish from the earth. "
The insertion is apparent in one of the surviving transcripts of the speech.