Why can't we have a world where people live and let live? What would be wrong with everyone just going about his/her business and letting everyone go about theirs? The answer is two-fold. First, no one can go about his/her business without making some kind of impact on many other people in the process. And, second, there is such a thing as truth.
Let's look at these two answers briefly. It is impossible for anyone to do much of anything that doesn't make some kind of impact on someone else. Think about it. What can you do that is entirely without impact on someone else? You can't buy anything without impacting a whole line of people--from the seller, to the manufacturer, to the parts supplier, to the raw materials producer, to the earthworm whose habitat is disturbed from the digging in the earth to find the raw materials. Likewise, you can't sell anything without directly impacting the person you sold to--as well as everyone who will be affected by the item you sold. You can't go anywhere without treading on someone's labors. You can't even die without putting several people out. Someone has to do the planning for your burial! No, there isn't much of anything you can do that is completely your own business. For that reason, no one can just live and let live because living means making some kind of impact on others.
Secondly, there is such a thing a truth. What I mean here is that there are some impacts that are good and wholesome and others that are wrong and destructive. The only people who are foolish enough to argue for the absence of moral boundaries are those who invest their lives in damaging others. If they were to receive the damage, we would hear about it quickly enough. For these kind of people, the only thing wrong is what hurts them. But, by their complaint they justify my basic premise: there is such a thing as truth, and good and evil.
With this is mind, it is not hard to see why we cannot simply live and let live. Someone has to insist that everyone do what is just and right. One of the reasons there is so much evil in the world today is that people have been too willing to hide their faces from the evil others do in the name of living and letting live. Someone has to say aloud and often, This is right and that is wrong.
That's why the churches exist. They do not exist to get along with each other. They do not exist to put up a false front of unity. They exist to call people to repent from evil doing and to exhort people to good works. But, here is the rub: you can't call evil people to repent without being called a nosy busy-body who needs to mind his/her own business! No evil person likes to be called evil.
Historically the church has always been persecuted because it stands for the truth. But these days no one wants to deal with the conflict that evil produces, so they say, We just want to live and let live. It can't be done. Everyone who tries to live that way ends up dying by degrees. So, in reality, living and letting live really means dying and letting die. What are you doing? Living or dying?