Here are a few reasons why I dislike the “Coexist” bumper-sticker.
The narrative of the bumper sticker implies that all violence and injustice in our world today is the result of religious difference and that religion is just a source of violence.  This is patently untrue.  How much violence and injustice in our world today is actually linked to greed, power and pride?  These are not the sole provenance of the religious person but rather the weakness of all human conscience.  The dictates of religion when authentically lived actually seek to curb these baser human tendencies.  It should also be noted that the twentieth century was the bloodiest in human history and the majority of blood spilled was by ideologies that were anti-religion.
The bumper-sticker implies that all these religious traditions are fundamentally the same and that any person who seeks to honor his or her tradition uniquely and live by its teachings is some form of extremist.  Again, this is not true.  Lived faith does not equate to extremism.  Respect for one’s own religious faith does not automatically mean a demeaning of another’s faith tradition.  It has been my experience over and over again that one of the hallmarks of the truly religious man or woman is a deep respect for the dignity of the other person and his or her beliefs.  I would even go on to say that religion truly lived gives access to a deeper and more profound respect for the human person than that which is possible through a bland secularism because through religion one can recognize the presence of the infinite in the other person – a reality that is deliberately denied through secularism.
The bumper-sticker seeks to establish a background narrative that people of different faith traditions cannot talk with one another nor get along.  Not true.  A number of times I have been able to attend the annual Prayer for Peace gathering organized by the Community of Sant’Egidio.  This annual event carries on the vision of Bl. John Paul II who brought together the leaders of all the world’s great religions to dialogue together and pray for peace according to the dictates of their own religion.  The Prayer for Peace does not propose a bland synchronism nor a diminishing (in Bl. John Paul’s case) of Christianity but rather an authentic living of one’s own faith as the true path toward encountering the other and the true path toward peace.
The bumper-sticker implies that peace can only be imposed on religions from without.  This is not true.  Any honest study and knowledge of the world’s great religious traditions will show that the seeds of peace and reconciliation are found within religious tradition and it is there that these seeds must be cultivated and are being cultivated.
Like the book and subsequent movie, “Eat, Pray, Love”; the bumper-sticker implies that any true and acceptable practice of religion in today’s world (if one must practice religion) will consist in a sampling and “picking-and-choosing” approach to religions and religious tradition that is more about confirming what I like and my preferences and ideologies rather than being challenged by a truth greater than me that will enable me to overcome my sinfulness and grow beyond my weakness.  
What surrounds the arrangements of the symbols of religion that make up the bumper-sticker is the bland, empty vacuum of a shallow secularism.  This is what we are left with when religion is diminished and derided.  The bumper-sticker in fact proposes a diminishment of the human person by seeking to truncate the capacity for religion and the desire for the transcendent.  Humanity is reduced (not achieved) when religion is reduced.  For full disclosure I will share that I am not a secularist nor do I find secularism appealing.  
Has great harm been done in the name of God and religion?  Yes, it has.  I am not seeking to deny this.  What I am seeking to say is that these acts of violence in the name of God are not the essence of religion and are, in fact, themselves a sin against religion and God.  To summarily equate religion with violence is itself an act of violence and disrespect.  It is also a profound act of ingratitude toward all the good that religion has done and continues to do in the lives of individual people and in the history and contemporary culture of our world.
Finally, to quote a college student at the Catholic Center where I minister who probably better summarized in one sentence the fallacy of the “Coexist” bumper-sticker than all that I have tried to share above.  “The ‘Coexist’ bumper-sticker says that all religions are the same and that they are all equally unimportant.”
I agree.