From what I understand the Creed surname likely owes it origin to Cornwall, England. From what I remember the thought was that the name came into being because of the town of Creed-on-Grampound and the name literally meaning "of Creed." The information that I read stated that when descendable surnames came into favor, the name attached to certain families from that area.

Cornwall was a gaelic speaking region and apparently there are variations of Creed from other gaelic speaking regions such as Ireland as well. It might be that it was a surname common to the gaelic speaking world.

Of course it is so far in the past who really knows...