The root of all good works is the hope of the Resurrection; for the expectation of the recompense nerves the soul to good works. For every labourer is ready to endure the toils, if he sees their reward in prospect; but when men weary themselves for nought, their heart soon sinks as well as their body. A soldier who expects a prize is ready for war, but no one is forward to die for a king who is indifferent about those who serve under him, and bestows no honours on their toils. In like manner every soul believing in a Resurrection is naturally careful of itself; but, disbelieving it, abandons itself to perdition. He who believes that his body shall remain to rise again, is careful of his robe, and defiles it not with fornication; but he who disbelieves the Resurrection, gives himself to fornication, and misuses his own body, as though it were not his own. Faith therefore in the Resurrection of the dead, is a great commandment and doctrine of the Holy Catholic Church; great and most necessary, though gainsaid by many, yet surely warranted by the truth. Greeks contradict it , Samaritans disbelieve it, heretics mutilate it; the contradiction is manifold, but the truth is uniform.
St. Cyril was technically semi-Arian (despite attempts to claim otherwise, this is really not in doubt), not in the sense that he disagreed with Nicene theology (he rejects Arianism and never says anything against Nicene theology), but in the sense that he seems to have favored compromises to make peace with people who were uncomfortable with the Nicene theology of homoousios, which was looked at by many as an unclear theological innovation breaking with the tradition of the Church. The Council of Nicaea had occurred in 325. It did not bring peace and clarity, but spread considerable confusion. 'Homoousios', the 'consubstantial' of the creed, was not a widespread term; a great many bishops, including many whose orthodoxy is not in dispute, thought that it was not helpful. Cyril became a bishop largely because he was a member of this group, and it is very noticeable that the corresponding Catechetical Lecture, despite being thoroughly orthodox, does not mention the homoousios at all.
A council was held in 360 in Constantinople which almost went Arian, but was pulled back from it by some complicated ecclesiastical politics. It did, however, protest the homoousios in moderate terms:
But since the term 'ousia', which was used by the fathers in a very simple and intelligible sense, but not being understood by the people, has been a cause of offense, we have thought proper to reject it, as it is not contained even in the sacred writings; and that no mention of it should be made in future, inasmuch as the holy Scriptures have nowhere mentioned the substance of the Father and of the Son.
This council had widespread acceptance. What seems to have brought it down ultimately was that a number of Arian-leaning bishops seem to have taken it as a license for much more aggressive action, which started pushing moderates like Cyril into direct opposition. When another council was held in Constantinople in 381, it went in favor of the homoousios, and it is the Creed of that council that is actually recited as the Nicene Creed. St. Cyril seems to have backed it completely this time; indeed, if some stories are to be believed, he is the one who actually wrote it -- the Creed of Constantinople according to some stories may well have been the baptismal confession of Jerusalem with the homoousios added. It's unclear (at best) whether this was actually the case, since we don't actually know how the Creed of the First Council of Constantinople came about (indeed, some have argued that the Creed of the First Council of Constantinople was actually not given by that Council, although the later Council of Chalcedon explicitly affirms it as such). But St. Cyril had certainly shifted his position.
Things did not get less confusing; most people ignored the First Council of Constantinople for quite some time afterward, although the council did mark a strong and permanent shift in the theological atmosphere (of which it may have been either cause or symptom). But Cyril's fight was coming to an end, and he died in 387.