When Bellamy wrote the original pledge--a poem--it did not contain the words "under God."  Those were added by President Eisenhower in the mid-50s, as you say, to differentiate us from the godless commies, but he did not have authorization from the author to bastardize his creation.  It would be akin to someone's changing a famous painting, and then trying to pass it off as authentic.  I believe Bellamy was a socialist AND an atheist, both of which were his constitutional right as an American.  Obviously, though, he felt patriotic enough to pen the poem.