It has been stated widely here and elsewhere that religion is an irrational pastime. This, by itself, is not enough to condemn it. People do lots of strange things- some of them are quite harmless. Religion, however, does not fall into the harmless category. It is causing immense harm on a world-wide scale, on both a personal basis, and to society as a whole.
Religion depends heavily on faith, so faith is promoted as a virtue, when it could in fact be seen as a sin. What should you call a mind set that requires no justification and brooks no argument? It is precisely this sort of thinking that can lead to terrorism. As Voltaire said long ago, "Those who can make you believe absurdities can make you commit atrocities."
Or as Nobel Prize-winning physicist Steven Weinberg put it, "Religion is an insult to human dignity. With or without it, you'd have good people doing good things and evil people doing evil things. But for good people to do evil things, it takes religion."
The three main monotheist religions have another nasty feature built in. Consider the following quote from Gore Vidal:
"The great unmentionable evil at the center of our culture is monotheism. From a barbaric bronze age text known as the Old Testament, three anti-human religions have evolved- Judaism, Christianity, and Islam. These are sky-god religions. They are, literally, patriarchal- God is the Omnipotent Father- hence the loathing of women for 2,000 years in those countries afflicted by the sky-god and his earthly male delegates."
This illustrates one of monotheism's worst features. By rejecting the god/goddess relationship, it has moved away from traditional spirituality. This male dominated religious viewpoint does not sit well with our spirituality instinct. That's why so many Christians tend to get hung up on the Virgin Mary and explains why the Egyptian goddess Isis has become a cult figure. It is an attempt to return to our spiritual roots.
Let's look at things from the standpoint of ethics. Religion is primarily based on fear and obedience, so it is no surprise that any ethic derived from religion will contain many "Thou shalt nots" and precious little positive advice on how to live your life.
For example, religion has been trying to stamp out sex for a very long time, with a singular lack of success. It has met its match here, for the sex drive is a much older and stronger instinct. What it can do though is cause large scale guilt among the faithful. This makes possible that most common of all religious journeys, the guilt trip.
Lack of self esteem is another leading cause of human misery, fostered and abetted by church attitudes regarding humility as a virtue and pride as a sin. Contrast this with Aristotle's doctrine of the golden mean, which places a proper sense of self worth as the virtue midway between the two sins of vanity and humility.
A religious mind is an emotional mind which tends to make decisions with an emotional bias, rather than using logic. It can't be any other way, because religion demands the suspension of our credibility, thus weakening logical thought processes. This means using the older more primitive part of the brain instead of the newer one, the neocortex.
As far as society on the whole is concerned, if you create a population of individuals suffering from various guilt complexes, make sure they feel bad about themselves so they will be humble, and get them to make most of life's decisions on an emotional poorly thought-out basis rather than a rational one, you will create a dysfunctional society. Lest you dismiss this as no more than a personal opinion, consider the following bit of research by US author and social scientist Gregory Paul, as published in the US academic Journal of Religion and Society which paints religion as society's biggest threat.
Paul says, "In general, higher rates of belief in and worship of a creator correlate with higher rates of homicide, juvenile and early adult mortality, STD infection rates, teen pregnancy and abortion in the prosperous democracies. The United States is almost always the most dysfunctional of the developing democracies, sometimes spectacularly so." He concludes, "The non-religious, pro-evolution democracies contradict the dictum that a society cannot enjoy good conditions unless most citizens ardently believe in a moral creator. The widely held fear that a godless citizenry must experience societal disaster is therefore refuted."
Finally, there is of course the possibility that religion will not only make societies dysfunctional, but extinct. These days, many people worry about radical Muslim terrorists. They should be equally worried about the Christian right's grip on the US government and its foreign policy.