Plausibly, ethics ought to be in part a practical discipline, one teaching us just how to become virtuous and avoid vice. In fact, this desideratum seems particularly fitting in the case of virtue ethics. I wonder how far ethics, and in particular virtue ethics, fulfills this.
In part, it does. I have learned some useful things from the Nicomachean Ethics about friendship. But in general, I rarely learn much that is useful for combating vice and pursuing virtue from secular philosophers, either ancient or modern. On the other hand, I learn a lot from Christian writers, both ancient and modern. These point out subtle dangers in the moral and spiritual life which I would not otherwise have noticed, and give useful advice on how to avoid these dangers and progress in virtue.
In part this is because many of the Christian writers are people who can draw on a rich experience of helping others, such as penitents, parishioners or spiritual directees, grow morally. This is a rich fund of data about the moral life which the secular philosopher has typically been completely bereft of. It seems clear that the secular moral philosopher who wishes to take seriously the therapeutic aspect of philosophy must study this fund of data.
But I also wonder if there isn't another reason for why secular ethics is only helpful to a point, a point that does not do much for one. For, ultimately, among fallen humans, moral progress is the work of the Holy Spirit, and in the moral life we contend not merely against what is human but also against the subtle intellects of demons.