Although groupthink or group mentality is divisive regardless of ideology, religion is a major contributor to divisiveness within society. Each group among the numerous cults of all religions are separate and adamantly so. Every separate group establishes a xenophobic nature where outsiders are concerned. Any outsider is looked upon with suspicion. Whereas secular groups are a bit less xenophobic, unless you adopt the ideological stances of the group you will be marginalized and ignored.
Although many religious cults have modernized many still hold to Iron Age beliefs. Superstition, ignorance, misandry, misogyny, and violence are inherent in these beliefs. I, personally, have observed misandric beliefs in some secular institutions.
In religious cults of various sorts faith is held as a virtue. Questioning the edicts of religion, no matter how nonsensical, is opposed vigorously. Science is regarded with respect only when it supports preconceived religious beliefs. Science is condemned when it contradicts conclusions arrived through religious means.
Religious groups claim to be benefactors to those in need. However, most simply use donated funds to further the growth of their specific faith rather than help the needy. Many times the services provided are dependent upon the acceptance of the principles of the faith. Those that do not meet the criteria are denied help.
Religious groups teach dependence. They teach a stoic acceptance of the status quo, rather than any attempt to alleviate suffering. The followers are taught that god will take care of their needs, and so, rather than take action they wait futilely.
As a result of religious groups believing that they know what is best for all, many seek power… in order to enforce their ideology. In many regards, the religious groups act like corporations for profit. They compete for market share. They use their money and power to influence governments. In their race for wealth and power the harm to society it might create is of no concern.