Thomas Aquinas, Compendium of Theology

To exclude the error of Ebion and Cerinthus, who taught that Christ's body was formed from male seed, the Apostles' Creed states: "Who was conceived by the Holy Ghost." In place of this, the Creed of the Nicene Fathers has: "He was made flesh by the Holy Ghost," so that we may believe that He assumed true flesh and not a phantastic body, as the Manichaeans claimed. And the Creed of the Fathers adds: "On account of us men," to exclude the error of Origen, who alleged that by the power of Christ's passion even the devils were to be set free. In the same Creed the phrase, "for our salvation," is appended, to show that the mystery of Christ's incarnation suffices for men's salvation, against the heresy of the Nazarenes, who thought that faith was not enough for human salvation apart from the works of the Law. The words, "He came down from heaven," were added to exclude the error of Photinus, who asserted that Christ was no more than a man and that He took His origin from Mary. In this heresy the false teaching that Christ had an earthly beginning and later ascended to heaven by the merit of a good life, replaces the truth that He had a heavenly origin and descended to earth by assuming flesh. Lastly, the words, "And He was made man," were added to exclude the error of Nestorius, according to whose contention the Son of God, of whom the Creed speaks, would be said rather to dwell in man than to be man.