Tonight, I gave my run-through of Christology up to Chalcedon in 2 h or less with a Greek translator. Whew! Bits may make it here as I reflect on things. In the questions at the end, one of my friends asked me a good question:
It seems that these guys spent a lot of time fighting against heresy, did they have anything to say about sin?
I think this is a great question because people like me (and, thus, the scholarly world at large) spend a lot of time discussing ‘the Fathers’ and heresy and how orthodoxy was forged on the anvil of heresy.
But what about sin, for St. Pete’s sake?
First, these people saw heresy and sin as intimately related. If you are an incorrigible sinner, you are probably a heretic. And if you are a heretic, you are probably a sinner.
Second, some of these heretical or non-mainstream (I don’t count Manichees as heretics but as members of an entirely different religion) groups engaged in what the (proto-)orthodox thought of as sin. Some Gnostics felt that what you did in the body didn’t matter, so they became gluttonous sex-aholics, basically. According to the report of Pope Leo I’s investigation into Manichaeism in the City, the Manichees were having ritual sex with underage virgins.*
Nonetheless, these people were concerned about holiness and sin, and not just my perenially-mentioned Desert Fathers. Augustine, for example, discusses in his Confessions that one of the reasons he delayed baptism was his enjoyment of pre-marital sex, and one of his falls after conversion was having vivid sexual dreams at night.
Since some people think Augustine was an over-guilty, Platonic, sexual deviant, I also encourage you to look at John Chrysostom’s sermons for their moral and ethical exhortations about things like lying or going to horse races or reading your Bible. Or take Augustine’s famous opponent Pelagius who was first targetted by the likes of Augustine for his moral rigorism.
Heresy is the doctrinal deviation of the human mind from God’s truth.
Sin is the moral deviation of human action from God’s path.
Both of them are matters of importance to the ancient Christians.
*For the full horror of this abomination, recall that a woman in the ancient world was ‘of age’ when she turned 12. I am actually cringeing having divulged that information.
Classically Christian 
“Some Gnostics felt that what you did in the body didn’t matter, so they became gluttonous sex-aholics, basically. According to the report of Pope Leo I’s investigation into Manichaeism in the City, the Manichees were having ritual sex with underage virgins.”
The same things were said of Christians by the Pagans. Anyone who doesn’t understand that these sorts of accusations have no historical relevance other than to demonstrate that different religions and even different sects in the same religion used such baseless accusations against each other as a matter of course back then, does not have any credibility with anyone who truly knows church history.
Now, the thing is, historically its been the case that Trinitarians are less concerned about sin while anti-Trinitarians are more opposed to it. Why is it, for instance, that Trinitiarians see nothing wrong with drinking, smoking, etc. and increasingly nothing wrong with pre-marital sex, whereas the “cults” like the Mormons and Jehovah’s witnesees (called cults simply because they are not trinitiarian, not because they are really any more authoritarian in forcing people to believe their stuff than the trinitarian churches are) all oppose these things vigourously? Why a little after the Reformation did all the Trinitarians abandom morality and yet the Socinians upheld it? It seems that when you truly believe Jesus is God, you tend to think Christology is all that matters and throw his teachings in the trash. Whereas when you think his purpose was mainly as teacher or prophet because he was a mere man appointed by God to teach us a certain message or “new law” then you put your emphasis on obeying his morality and hence on fighting sin rather than ‘heresy.’ Heretics fight sin; orthodox fight heretics precicely to defend sin.
For most of its history, Trinitarian Christianity has beend anti-drunkenness, and many Trinitarian Christians are anti-smoking, as well. Furthermore, most Trinitarian Christians who have a traditional, conservative view of the Bible and doctrine also believe in saving sex for marriage. What has corrupted the Trinitarian church in these ways is nineteenth-century liberalism, which is actually an attack on traditional biblical beliefs in more areas than simply morality and ethics — although it currently is attacking these as well. The Orthodox, Roman Catholics, and most evangelical groups do not believe in drunkenness, smoking, or pre-marital sex. They also believe in not manipulating people to be moral. But if you listen to the sermons of many preachers from those groups, you will see that they do, in fact, take a strong, moral stand.
That the Protestant mainline has abandoned much of traditional morality is a sad reality but is also not a result of being Trinitarian.
If we look at the high moral standards of ancient Christians through to the central Middle Ages, and many cases beyond despite the corruption of the institutions of the church, we will see that they, too, had very high and rigorous moral standars. What you say about SOME modern, Trinitarian liberals is true. But it is not reflective of the vast majority of Christian history. I recommend you read the sermons of Trinitarian preacher John Chrysostom, preferably the expository ones. Also of worth as regards morality is the Venerable Bede. Then you will see both an eastern and a western Christian and the high moral standards they expected of believers. Indeed, early Medieval Trinitarian believers in England believed in morality so strongly that they believed the Viking raids in 793 and the Norman Conquest in 1066 to be God’s wrath against them for their sin. We may reject the viewpoint that God works that way today, but the fact is, they were aware of their sin.