Whether or not religion adds to divisiveness between religions, or the faithful and atheists is anyone's guess. We don´t really have a control group. But I´d say it´s beyond question that religion decreases xenophobia between people who have the same religion. Isn´t that a good thing?
I don't see it as a guess, there are enough examples of religious intolerance, (including our understanding of human nature), to support the proposition that there is always going to be certain percentage of people in every religion who show their intolerant of other religions, denominations, etc, Sunni's and Shiites, Muslims and Christians and so on.
For
example:
''However, the relationship between religion and conflict is, in fact, a complex one. Religiously-motivated peace builders have played important roles in addressing many conflicts around the world. This aspect of religion and conflict is discussed in the parallel essay on religion and peace. This essay considers some of the means through which religion can be a source of conflict.''
''Some groups, such as America's New Christian Right and Jama'at-i-Islami of Pakistan, have operated largely through constitutional means though still pursue intolerant ends. In circumstances where moderate ways are not perceived to have produced results, whether social, political, or economic, the populace may turn to extreme interpretations for solutions. Without legitimate mechanisms for religious groups to express their views, they may be more likely to resort to violence. Hizbullah in Lebanon and Hamas in Palestine have engaged in violence, but they also gained supporters through social service work when the government is perceived as doing little for the population. Radical Jewish cells in Israel and Hindu nationalists and Sikh extremists in India are other examples of fundamentalist movements driven by perceived threat to the faith. Religious revivalism is powerful in that it can provide a sense of pride and purpose, but in places such as Sri Lanka and Sudan it has produced a strong form of illiberal nationalism that has periodically led to intolerance and discrimination.[1] Some religious groups, such as the Kach and Kahane Chai parties in Israel or Egypt's Islamic Jihad, consider violence to be a ‘duty'.[2] Those who call for violence see themselves as divinely directed and therefore obstacles must be eliminated.''
''Many religions also have significant strains of evangelism, which can be conflictual. Believers are called upon to spread the word of God and increase the numbers of the flock. For example, the effort to impose Christianity on subject peoples was an important part of the conflict surrounding European colonization. Similarly, a group may seek to deny other religions the opportunity to practice their faith. In part, this is out of a desire to minimize beliefs the dominant group feels to be inferior or dangerous. Suppression of Christianity in China and the Sudan are but two contemporary examples. In the case of China, it is not a conflict between religions, but rather the government views religion as a dangerous rival for citizens' loyalties. All of these instances derive from a lack of respect for other faiths.''
Although the article concludes with - ''in the eyes of many, religion is inherently conflictual, but this is not necessarily so'' - I don't think that this is a fair conclusion, or assumption, given the inherit divisiveness of faith based beliefs, which do in fact conflict with each other in terms of the articles of belief, be they political, ideological or religious.