Often the atheist will argue that belief is not necessary, what is necessary is being a good person, someone who will respect others and not injure them. This he has in common with the liberal, who believes that the only purpose of government is to prevent people from hurting each other. Indifferentists will argue that all religions are the same, that it doesn't matter what one believes, as long as one is a good person and does good things. According to them, the primary purpose of religion is to teach people to follow the Golden Rule.
What of the virtues with regard to the self? It seems that most will not acknowledge these, believing that one is free to do whatever one wants with regards to onself, as long as one does not injure someone else in the process, or use someone else as a "means to an end."
In one way, they are correct--with regareds to political life, the virtue of justice is the summit of virtue. But it isn't the greatest virtue, and with respect to our moral formation, it is perhaps easier to grasp than the other moral virtues. At least the habit of not harming others can be inculcated rather easily, as it can be backed up with punishment. The demands of justice are easy to explain--to understand how we are to behave towards others, we can reflect upon how we want to be treated.
Once we understand that other people are like us, and how we can be harmed, we know what can harm others.
The precepts of the moral virtues that pertain to the self, like temperance, can be more difficult to recognize, especially if we have not been brought up with good habits. Once reason has been corrupted, if we become slave to our sense appetites, it will be difficult to convince us that a virtue of temperance is possible or worth pursuing.
More importantly, religion is a form of justice, rendering what is due to God, and not all belief systems are the same in this regard. If we receive existence from some source, we owe something to that source. So those who claim to put a premium on justice are actually neglecting the most important form of justice.
[If there had been no sin… in a created ordered directed to a natural end, would God send his angels to punish sin and to save men?]