Religion can divide and segregate people, depending upon dogma. Many religions see "their way" as the only way and in doing that segregate themselves from others who believe differently from them. Yet, religion can also give comfort to many who believe and have similar faith. Then there are religions that welcome all people regardless of their belief systems.
If the idea of religion is to be a set of beliefs and practices based upon a faith in a supernatural deity, it can be both divisive and welcoming, depending upon whether people believe in the traditions and practices. But if the religion allows that there are other beliefs and faiths that may be valid, then the result will not be divisive. It is quite possible for people to coexist in this world and have differing religions. After all, the variety of beliefs in the world merely provides a basis for examining similarities.
Almost all religions are monotheistic. All of the religions under the umbrella of Christianity are very similar. The divisiveness that appears in religion comes from the idea that one is better than the other. Yet, in the Bible, Jesus spoke against judgment of others. While some may say that the belief that their religion is better than others, or the way to be saved, it is really a way of judging the beliefs of others. Judgment begets divisiveness and segregates people.
Religion can be a guide to living a good life, with good morals. Yet some use it to wage hate rather than waging peace and unity. For some it is a passport to do whatever they want and not be accountable in their religious life. After all, if one repents for their wrong doings they are forgiven. Excuses that promote hate and intolerance and lack of moral conscience do nothing but alienate people who are seeking a genuine, loving deity.
Religion that is tolerant and sees people as "works in progress" and welcomes them despite their faults is able to unify and provide guidelines that allow individuals to grow beyond what they might otherwise. The world needs this kind of religion, not religion that is based upon divisive excuses that allow fanatics to hate and be sanctioned by God, Allah or any other god.
In conclusion, religion can be either unifying or divisive. It is up to each individual religion to determine whether they want to be a unifying force in the world community or a vehicle of alienation. For the world to become a more loving and peaceful place, alienation and segregation of members of various religions needs to end. Standing together as a family of humanity is our only hope.