The idea that Christians should "hate the sin but love the sinner" is a popular defense of Christian attacks on homosexuality, abortion, and other behavior which they consider abhorrent. Critics rightly point out that hating the sin can quickly become hating the sinner as well, something which many Christians appear to indulge in when it comes to gays, for example. Is this a problem with atheist critics of religion can succumb to as well?
In Humani Generis Unitas (The Unity of the Human Race), the so-called "lost encyclical" against anti-Semitism and racism which Pope Pius XI was unable to edit and deliver due to his death, there is the following statement:
Zeal against the sin readily becomes zeal against the sinner; but zeal against the sinner soon throws off its mask and shows itself for what it really is, an assault, under the pretense of protecting society from a single social group, upon the very basis of society, an evocation of limitless hatred, a license for every form of violence, rapacity, and disorder, and an engine against religion itself.
Some of irreligious atheists who criticize religion engage in something analogous to "hating the sin but loving the sinner" when it comes to hating the "sin" of irrational, unfounded religious beliefs but maintaining respect and consideration for the believers themselves. Atheists, however, are not inherently better human beings than Christians, so if it's possible for Christians to go from hating the sin to hating the sinner, then it's possible for atheists to do so as well.
Atheists Should Learn from Pope Pius XI
Irreligious atheists should, then, learn from the example of conservative Christians and understand that whether "sin" is meant literally or metaphorically because too much zeal against an idea, opinion, position, belief, or tradition can easily become zeal against the people who hold and defend it. People deserve some basic and automatic respect while ideas or beliefs do not, but whereas the distinction between the two isn't too hard to make in theory it can be easy for it to be lost in practice — and especially in the heat of arguments.
This cannot be treated as a reason not to criticize religion, argue against religion, disagree with religious believers, consider religion harmful or silly, or even to be zealous against religion and religious beliefs. If it were, then the same would apply to just about every other area of disagreement because any time people zealously pursue one idea against another, they runs the risk of dropping the distinction between ideas and the people who hold them.
It also isn't necessarily a reason for atheists to be more "moderate" in their criticisms and more "tolerant" of what they consider the excesses or falsehoods of religion. If a person considers some belief or belief system to be wrong, irrational, or even dangerous, they shouldn't refrain from making strong arguments on behalf of their position simply because it's possible that their arguments could go too far if they aren't careful. Once again, the same would end up applying to every area of disagreement and no one would support such a development.
What this does mean is that irreligious atheists should be careful and fair when they are criticizing religion. Religion, religious beliefs, religious ideas, religious doctrines, and religious opinions are all fair game for whatever criticism a person thinks is justified, but critics must take care not to frame their critiques in a way that unfairly attacks believers themselves. As much as possible, the focus must be on beliefs and ideas rather than people and personalities. When a critic strives to maintain this distinction, they are less likely to fall into the trap hating people simply for believing and thinking differently.