No hay evidencia alguna de que Magallanes haya dicho esto, siendo la única referencia un ensayo de Robert Green Ingersoll en que dice «Creo que fue Magallanes quien dijo [...]».
Fernando de Magallanes Frases y Citas
Fernando de Magallanes: Frases en inglés
This quotation is often found on the internet attributed to Magellan, but never with a source, and no English occurrence prior to its use by Robert Green Ingersoll in his essay "Individuality" http://www.infidels.org/library/historical/robert_ingersoll/individuality.html (1873) has been located. Thus, it it most likely spurious. In that essay Ingersoll states:
It is a blessed thing that in every age some one has had individuality enough and courage enough to stand by his own convictions, — some one who had the grandeur to say his say. I believe it was Magellan who said, "The church says the earth is flat; but I have seen its shadow on the moon, and I have more confidence even in a shadow than in the church." On the prow of his ship were disobedience, defiance, scorn, and success.
Disputed
Variante: The Church says that the Earth is Flat, but I know that it is Round. For I have seen its Shadow on the Moon and I have more Faith in a Shadow than in the Church.
Fuente: As quoted in Oxford Academic (25 July 2013) http://oupacademic.tumblr.com/post/56463634957/misquotation-i-have-seen-the-shadow-of-the-earth