There are a tonne of resources around that compare the world's major religions, highlighting the differences between each. There are some good comparisons of Eastern vs Western religions, and also numerous comparisons of Christianity vs non-Christianity.
However, I wasn't able to find any articles that specifically investigate the compatibility between the world's major religions. The areas where different religions are "on the same page", and are able to understand each other and (in the better cases) to respect each other; vs the areas where they're on a different wavelength, and where a poor capacity for dialogue is a potential cause for conflict.
I have, therefore, taken the liberty of penning such an analysis myself. What follows is a very humble list of aspects in which the world's major religions are compatible, vs aspects in which they are incompatible.
- Divinity (usually although not universally manifested by the concept of a G-d or G-ds; this is generally a religion's core belief)
- Sanctity (various events, objects, places, and people are considered sacred by the religion)
- Community (the religion is practiced by more than one person; the religion's members assemble in order to perform significant tasks together; the religion has the fundamental properties of a community – i.e. a start date, a founder or founders, a name / label, a size as measured by membership, etc)
- Personal communication with the divine and/or personal expression of spirituality (almost universally manifested in the acts of prayer and/or meditation)
- Stories (mythology, stories of the religion's origins / founding, parables, etc)
- Membership and initiation (i.e. a definition of "who is a member" of the religion, and defined methods of obtaining membership – e.g. by birth, by initiation ritual, by force)
- Death rites (handling of dead bodies – e.g. burial, cremation; mourning rituals; belief in / position regarding one's fate following death)
- Material expression, often (although not always) involving symbolism (e.g. characteristic clothing, music, architecture, and artwork)
- Ethical guidance (in the form of books, oral wisdom, fundamental precepts, laws, codes of conduct, etc – although it should also be noted that religion and ethics are two different concepts)
- Social guidance (marriage and family; celebration of festivities and special occasions; political views; behaviour towards various societal groups e.g. children, elders, community leaders, disadvantaged persons, members of other religions)
- Right and wrong, in terms of actions and/or thoughts (i.e. definition of "good deeds", and of "sins"; although the many connotations of sin – e.g. punishment, divine judgment, consequences in the afterlife – are not universal)
- Common purpose (although it's impossible to definitively state what religion's purpose is – e.g. religion provides hope; "religion's purpose is to provide a sense of purpose"; religion provides access to the spiritual and the divine; religion exists to facilitate love and compassion – also plenty of sceptical opinions, e.g. religion is the "opium of the masses"; religion is superstition and dogma for fools)
- Explanation of the unknown (religion provides answers where reason and science cannot – e.g. creation, afterlife)
- The nature of divinity (one G-d vs many G-ds; G-d-like personification of divinity vs more abstract concept of a divine force / divine plane of existence; infinite vs constrained extent of divine power)
- Acknowledgement of other religions (not all religions even acknowledge the existence of others; of those that do, many refuse to acknowledge their validity; and of those that acknowledge validity, most consider other religions as "inferior")
- Tolerance of other religions (while some religions encourage harmony with the rest of the world, other religions promote various degrees of intolerance – e.g. holy war, forced conversion, socio-economic discrimination)
- Community structure (religious communities range from strict bureaucratic hierarchies, to unstructured liberal movements, with every possible shade of grey in between)
- What has a "soul" (all objects in the universe, from rocks upward, have a soul; vs only living organisms have a soul; vs only humans have a soul; vs there is no such thing as a soul)
- Afterlife (re-incarnation vs eternal afterlife vs soul dies with body; consequences, if any, of behaviour in life on what happens after death)
- Acceptable social norms (monogamous vs polygamous marriage; fidelity vs open relationships; punishment vs leniency towards children; types of prohibited relationships)
- Form of rules (strict laws with strict punishments; vs only general guidelines / principles)
- Ethical stances (on a broad range of issues, e.g. abortion, drug use, homosexuality, tattoos / piercings, blood transfusions, organ donation)
- Leader figure(s) (Christ vs Moses vs Mohammed vs Buddha vs saints vs pagan deities vs Confucius)
- Holy texts (Qu'ran vs Bible vs Torah vs Bhagavad Gita vs Tripitaka)
- Ritual manifestations (differences in festivals; feasting vs fasting vs dietary laws; song, dance, clothing, architecture)
Why can't we be friends?
This quick article is my take on the age-old question: if all religions are supposedly based on universal peace and love, then why have they caused more war and bloodshed than any other force in history?
My logic behind comparing religions specifically in terms of "compatibility", rather than simply in terms of "similarities and differences", is that a compatibility analysis should yield conclusions that are directly relevant to the question that we're all asking (i.e. Why can't we be friends?). Logically, if religions were all 100% compatible with each other, then they'd never have caused any conflict in all of human history. So where, then, are all those pesky incompatibilities, that have caused peace-avowing religions to time and again be at each others' throats?
The answer, I believe, is the same one that explains why Java and FORTRAN don't get along well (excuse the geek reference). They both let you write computer programs – but on very different hardware, and in very different coding styles. Or why Chopin fans and Rage Against the Machine fans aren't best friends. They both like to listen to music, but at very different decibels, and with very different amounts of tattoos and piercings applied. Or why a Gemini and a Cancer weren't meant for each other (if you happen to believe in astrology, which I don't). They're both looking for companionship in this big and lonely world, but they laugh and cry in different ways, and the fact is they'll just never agree on whether sushi should be eaten with a fork or with chopsticks.
Religions are just one more parallel. They all aim to bring purpose and hope to one's life; but they don't always quite get there, because along the way they somehow manage to get bogged down discussing on which day of the week only raspberry yoghurt should be eaten, or whether the gates of heaven are opened by a lifetime of charitable deeds or by just ringing the buzzer.
Religion is just one more example of a field where the various competing groups all essentially agree on, and work towards, the same basic purpose; but where numerous incompatibilities arise due to differences in their implementation details.
Perhaps religions could do with a few IEEE standards? Although, then again, perhaps if the world can't even agree on a globally compatible standard for something as simple as what type of electrical plug to use, I doubt there's any hope for religion.