I think that religion has been a force of peace more than it has been a force of unrest. While religion may have been the cause of a lot of wars in the past, as well as a lot of problems in today's world, I still think that it has helped a lot of people as well.
I believe more often than not, religion is a force of unrest. We can see this throughout history as people never seem to agree over large populations, what is right. Humanity has a serious problem with religion because we feel the need to push our beliefs on other people, which is absolutely wrong. There are some religions which have totally avoided this problem, so it is possible.
I don't think religion has ever been a force of peace. Looking back through history you see aggression, wars, atrocities and upheavals, all in the name of one religion or another. Today we have border wars, nations threatening other nations, and charges of sexual improprieties - all due to religion. I don't see how we could refer to religion as a force of peace.
The institution of organized religion has holes; I distrust the infrastructure. They try to control how its members think and perceive various concepts about the world rather than allowing individual interpretation (fundamentalism being the heaviest on power and control, I also feel as though the Catholic Church is about control). Spirituality should be personal. I also mistrust organized religion due to its tendency to drive people to commit atrocities.
There has been no force equally as brutal as religion in humanity's short history. If you look at almost every war, there has been a religious aspect to it. Then you have the crusades, the inquisition, suicide bombings, flying jets into planes. Not to mention how badly women are treated.