I like the Zen concept that opposites are fundamental in the universe. The ideas of wall and window or door do not mean anything without the other one. A window is an absence of wall. Without walls there could not be windows.
People tend to get mad very quickly when you point out similarities between their religion and others, but Christianity in theory is very similar to Buddhism in theory. Jesus, like Buddha, recommended abandoning material wealth and respecting life. Jesus also frequently used metaphors that seem very Zen to me. "Sometimes the master is also the servant" is one that comes to mind. But no one wants to hear this. They get too mad. The Christians are like "how dare you compare us to the heathens" and the Buddists are like "how dare you compare my enlightenment to that of Ned Flanders." Religion seems to make people with very similar philosophies hate each other for trivial reasons. The various sects of Islam murder each other over differences in historical doctrine that I can barely understand. They are fighting about things that happened in the time of Abraham and Noah. So similarities in religion are a touchy subject, but most of the world religions share the same basic principles: don't lie/murder/steal/hurt people, care about others, material wealth does not buy spiritual happiness, etc.
