Legally, creed is a synonym of religion. To establish a difference in meaning it would be necessary to find a law that uses both terms and investigate why the law uses them both, but I have not found such a law. My search was not particularly comprehensive, but the laws that it found used only one word or the other. If you know of a law that uses both terms, please post a comment.
The term creed derives from the Latin credo, meaning I believe, because creeds typically start that way (for example, "I believe in the Great Pumpkin..."). The culture and laws of the United States, along with the English language, arose in explicitly Christian kingdoms (England and the UK, and, to a lesser extent France, with some influence from Spain), so it is instructive to consider the history of creeds in Christianity, in the sense of a statement of faith.
The Christian creeds are products of ecumenical conferences that fixed their texts for the purpose of establishing orthodoxy. This is small-O orthodoxy, meaning the set of beliefs that defines the religion. For example, the Nicene Creed was established to refute the theological doctrine of Arius. That is, the ecumenical council of Nicaea declared that someone who does not believe the things stated in the creed is not following the Christian religion.
Of course, that declaration doesn't stop someone with unorthodox beliefs from claiming to be a Christian. More generally, it is not uncommon for two religious groups to hold conflicting beliefs about their religion, and each may claim that adherents of the other are not "true believers" or indeed that they are not members of the same religion.
This brings us back to the use of "creed" to define protected characteristics. In this sense creed does not necessarily denote the statement of beliefs itself but rather the system of beliefs being stated. As mentioned in the thesis paragraph, a (fairly brief) search did not find a law that prohibits discrimination both on the basis of creed and of religion; the laws I found used only one term or the other. I suspect that creed could be used to underscore the fact that the law also protects against discrimination between different groups within the same religion. However, it is easy to argue that such discrimination is also based on religion, so, in the legal sense, creed is a synonym of religion.