The Similarities in All Major World Religions

One Ultimate.
This is always present in a belief system:
A singular being superior to mankind, or more than one being superior to mankind. In the case of a system having more than a dozen deities, there will be one that is dominant in some way, or there will be a hierarchy. If the singular omniscient being is not a being, it is represented somehow else; The Truth, Enlightenment, Oneness, Ultimate Reality, The Universe, Completion, Absolution, Perfection, whatnot. In the case of a god, this god is often merciful, forgiving, righteous, perfect, etc.; but s/he / they will also damn you to burn in hell, die, suffer for all eternity, suffer temporarily, grant you bad karma/dharma , or something similar. If you ask for forgiveness, or complete a purification ritual of some sort, you will be forgiven.

Play Nice, Be Nice.
In all religions, even ones considered to be "bad", you are instructed to do several things:
Keep promises, don't lie, don't steal, help people, take care of yourself, don't hurt people; attend a place of worship regularly, or worship regularly, or otherwise frequently affirm your faith. By praying, meditating, asking for forgiveness, repenting, etc., you give praise to the Higher and improve your standing with the Higher and/or come closer to being with the Higher.

All religions encourage good behavior, or moral behavior. Morality is generally taken as per the general meaning of 'good', but, in some rarified other religions, 'bad' is taken as the 'right' morality. I won't comment. See this: In Judaism, see Leviticus 19:18 NIB. "What is hateful to you, do not do to your fellowman." In Christianity, see Luke 6:31 NIB. "Do to others as you would have them do to you." In Islam, see a hadith recorded by al-Bukhari, Sunnah: "No one of you is a believer until he desires for his brother that which he desires for himself." In Hinduism: Mahabharata 5,1517: "Do naught onto others what you would not have them do unto you." Buddhism's Udana-Varga 5,18: "Hurt not others in ways that you yourself would find hurtful."

Of course, there are many others. See Taoism, Jainism, Confucianism, Zoroastrianism, Baha'i, Wicca, Old Egyptian faiths, and many others.

Life After Death, and Rewards.
Either eternal Heaven, or eternal Hell. If you're good in life, you'll be rewarded in death. If you're bad in life, you're punished in death. In Eastern religions, it's usually reincarnation, or a form of reincarnation. You assume the form of another being after you die, usually with no memory of what you once were. Sometimes you'll be reincarnated many times until you reach perfection, or something similar. In some beliefs, depending on how you behaved in life, you will reincarnate as either another person (being the best possible outcome next to Perfection) or you will become an animal (not as good, but not the worst), or you will become some lesser creature (the weakest creatures, or the ones with the least senses, or apparent sentience, are those that were the worst.).

One Mother, One Origin.
In many religions, one man and/or woman or one group of people were created first, and from them, came the rest of mankind. Usually organized as tribes, clans, or something similar. Tribal lands were eventually either torn into sections or consolidated between families, depending on feuds and conflicts. These lands became the countries we know today.
In many religions, the story goes, that at one time, everyone spoke one language, but mankind did something to incite the anger of the Higher, and the languages were scattered, and mankind stopped being able to communicate for a while.

A Messiah, or a Sacrifice; A 'Lamb', or Ritual 'Lambs'.
In most cases, there is a singular or multiple case or cases of sacrifice by either the Higher to Man or from Man to the Higher, in which Man achieves the forgiveness of the Higher for wrongs done. In the case of Christianity, the Higher (God) made a sacrifice (Jesus Christ) in order to save Man from sin. This singular sacrifice, in some cases, becomes, instead, a series of ritual 'sacrifices', which aren't actual sacrifices, but, actually, may be ritual bathing, ritual praying, or something similar.

The End of Days.
It's just about unanimous that, at the end of the world, there will be a great battle between good and evil, and Final Judgment will be made. Several religions indicate the coming of a prophet, a messiah (in the case of Christianity, the Second Coming of Christ), or something or someone similar.

With all these similarities, and with the Play-Nice ideal in mind, why can't we all just stop bombing each other? Obviously, it's because of minor idealogical differences, but we all believe, essentially, the same thing (unless you're an atheist; in which case, this isn't about you).