This might seem like a petty topic and maybe it is, but for anyone who's given any thought on the underlying assumptions of these bumper stickers, it's just annoying. If you just bought a used car with one of these stickers on it, scrape that thing off! Nevermind the tackiness a bumper sticker creates on an otherwise decent vehicle, but announcing your political, religious, or virtuous standings is a recipe for vandalism. Keep in mind, most people are away from their car more than they're with them, making them easy targets. So, don't give someone the motivation because there will be plenty of opportunity.
However, if you insist on displaying your thoughts on a complicated issue in an oversimplified way, you'll have people like me behind you, judging you, making assumptions about you, questioning your cognitive faculties, and wondering if your yard is littered with pink flamingos, gnomes, bird baths, and toilet bowl flower pots. You'll also have people like me blogging about people like you. So, let's get to it.
First, let's discuss those underlying assumptions. It seems like those who plaster the "coexist" bumper sticker on their car are making an admirable appeal to peace, something we all long for. But, the sticker also implies a few different things, like: 1) competing religions are combative, 2) that all religions are false, so, in the words of Rodney King, "why can't we all just get along?", or conversely, 3) all religions are true, so "why can't we all just get along?"
1) A Combative History of Religion
A moment of concession: there's no denying that competing religions have been combative throughout history. Many religions have a history of war and violence. 9/11 was religiously motivated, the Crusades, the Spanish Inquisition, etc., are all scars on religious history. However, let's also dispel with the myth that religion has caused most wars. The Encyclopedia of Wars, a three volume collection of ancient, medieval, and modern wars, documents a total of 1,763 wars, 121-123 of which were considered religiously motivated. That means, of all of the wars documented, religiously motivated wars contributed to approximately 7% of that total. Subtract the Islamic wars and all other religiously motivated wars drops to nearly 3%. Whether 3% or 7%, it's, admittedly, 3% to 7% too many. But a far cry from causing the most wars.
Perhaps, however, the assumption is that competing religions are simply disrespectful or intolerant of each other. As a Christian myself, my experience of disrespect has been primarily at the hands of non-believers, usually consisting of ridicule or derisive comments. This is something that one of the chief proponents of modern atheism, Richard Dawkins, has encouraged his followers, to "mock them. Ridicule them in public". There is, however, a tendency to conflate disagreement with disrespect. One can certainly disagree with a view without being disrespectful, something I try to do. Disagreeing with a view can also be confused with being intolerant of a view. But, to tolerate a view presupposes you disagree with it. After all, if you agree with a view you don't "tolerate" it, you agree with it!
2) Are All Religions False?
That's a bold assumption. It's an assumption that likely ignores the centuries of ongoing philosophical argumentation, like: the ontological argument, the Leibnizian cosmological argument, the Kalam cosmological argument, the teleological argument, the moral argument, etc. Religious discussion and debate is very much alive within academia with believers being some of our most respected scholars, professors, and professionals in top institutions and positions. As an example of that lively discussion, check out one my favorite debates between Dr. William Lane Craig and the late Christopher Hitchens. Now, I don't expect everyone to be philosophically well versed, as unfortunate as that may be, so ignorance is expected and understandable. But for anyone who thinks all religions are false or that "God is dead", they just haven't been paying attention.
3) Are All Religions True?
This is likely the more common assumption, but it's equally as bold for the fact that it's not actually possible. All religions can't be true because all religions are mutually contradictory. Within logic you have the Law of Non-contradiction that states, "contradictory propositions cannot both be true in the same sense at the same time". As you'll see, to suggest that all religions are true is to violate the law of non-contradiction.
Take, for instance, just the conceptual differences of deities between monotheism, polytheism, and pantheism. As their titles indicate, monotheistic religions (e.g. Judaism, Christianity, and Islam) believe in only one God. Polytheistic religions (e.g. Hinduism, Greco-Roman religions, Egyptian religions, etc.) believe in a plethora of gods. Pantheistic religions (e.g. some Hindu and Buddhist doctrines) believe that the universe and everything within is God or divine. In these situations, you have either "only one God" or "not only one God", but you can't have both. If we were to include atheism or non-theistic religions in our bag of worldviews, then we have another alternative to consider, "there is no God or gods". Hopefully that illustrates why all religions (or worldviews) can't be true, because they are mutually contradictory.
Consider the mutually contradictory views within monotheism alone. Judaism and Islam view God as unitarian. Whereas, Christianity views God as trinitarian. Consider Christianity's view of Jesus as the second person of the trinity, who was crucified and resurrected. Whereas, Islam views Jesus as a mere prophet, whose crucifixion was falsified, so a resurrection never occurred. Consider the Jewish and Christian view that Isaac, the son of Abraham, was the promised son, whose descendants, the Israelites, would become the chosen people of God. Whereas, Islam views Ishmael, Abraham's first son, as the promised son, whose descendants, the Arab people, were God's chosen people. If you throw deism into the mix, then you have an impersonal God instead of the personal God affirmed in Judaism, Christianity, and Islam. Obviously, even across monotheistic faiths, they can't all be true because they contradict each other.
In short, all religions (or worldviews) can't be true, but they could all be false. Obviously, as a Christian myself, I believe Christianity is true. So, I don't believe all religions are false. With that being said, Jesus made it very clear in John 14:6, "I am the way and the truth and the life. No one comes to the Father except through Me". That's a pretty exclusive claim. If the life, death, and resurrection of Jesus is true and we are to believe what Jesus taught, that means Christianity is the only true religion. You might find that to be offensive or bigoted, but it's certainly not a unique claim. Many religions or worldviews make exclusive claims, even those that claim to be inclusive.
The moral of the story, however, if I haven't made it clear already, is your "coexist" sticker needs to go.
Consider this one instead.
