A modern view of virtue ethics can be contrasted with the classical view. Classically, the concept of arete (Greek for virtue) is in reference to goals. A thing is virtuous if it is good for its purpose. Aristotle's metaphysics saw four real causes of things, the formal (what it is eg the human soul), the material (what it is made out of, eg the human flesh), the efficient (what causes it to be, eg the intercourse of the parents), and the final (what it is for, eg the meaning of human life). For Aristotle and other classical virtue ethicists, if one knows the final cause of man, then one can enumerate the virtues of man. This gives an objective fact the man's virtue. Whatever virtues point towards the final end of man, a man is obligated to cultivate these, to build his character such that he is suited for and oriented towards his purpose. Then, he will live the good life by just following his habits. Those virtuous (or vicious) habits are his character.
By contrast, Anscombe and the rest will reject that man has a final end as long as they are not theists. Modern philosophy tends to reject the fourth cause, and in many cases also rejectes the first (the formal cause). This is because many reject metaphysical realism. The material cause is implicitly acknowledged, but little thought is given to it. The dominance of natural science has led us to a hyper-focus on the efficient cause.
But, with the rejection of final causality, the virtue ethicist throws out any justification for universal obligation. If man has no final end, then he is free to choose his own purpose. Or, perhaps, society chooses it for him. This is why the IEP says of MacIntyre:
Each account of virtue requires a prior account of social and moral features in order to be understood. Thus, in order to understand Homeric virtue you need to look its social role in Greek society. Virtues, then, are exercised within practices that are coherent, social forms of activity and seek to realize goods internal to the activity. The virtues enable us to achieve these goods.
The very next sentence, here, says also for his philosophy
There is an end (or telos) that transcends all particular practices and it constitutes the good of a whole human life. That end is the virtue of integrity or constancy.
MacIntyre only believes this because he is Catholic. Because he is Catholic, he believes that God is man's final end. Specifically, eternal contemplation of the Divine, sometimes called by Christian theologians the Beatific Vision. Therefore, MacIntyre has a cause for universal obligation; man has a universal duty to cultivate those virtues which will aid him in participation in the Beatific Vision. But for the atheistic or post-Christian virtue ethicist, virtue can only be a relative thing, personally chosen or received from society. Bernard Williams, also mentioned in the IEP article, is an example of an atheistic virtue ethicist.