You got me on that one:

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Wasn't the Nicene Creed written in response to the Arian heresy?

Indeed it was. It takes the form of a statement describing the nature of Jesus Christ as both fully God and fully man. There has been no need to modify it, though, because it is a positive statement of the truth more than a negative statement against things which are not true. I think that's one of the most profound strengths of the great creeds and confessions; that they are positive assertions rather than argumentative descriptions and refutations of false doctrine.

-Robin