Athanasian Creed: taking of the Manhood into God

The Athanasian Creed was an expression of western Nicene-Chalcedonian belief, including the hypostatic union. But there is one sentence where the intention is not as obvious to me as the rest: > ... unus est Christus. Unus autem non conversione divinitatis in carnem, sed assumptione humanitatis in Deum. which the Book of Common Prayer translated as: > ... one Christ; One, not by conversion of the Godhead into flesh: but by taking of the Manhood into God; Which particular misunderstanding or heresy was this particular distinction aimed at? How is it read with John 1:14 "And the Word became flesh ..." or even the plain etymology of *incarnation*?