For the commandments, "You shall not commit adultery," "You shall not murder," "You shall not steal," "You shall not bear false witness," "You shall not covet," and if there is any other commandment, are all summed up in this saying, namely, "You shall love your neighbor as yourself."
Romans 13:9 NKJV
At the heart of every religion is one core rule. We have come to know this as the Golden Rule and it has been said many times by many of the great spiritual leaders.

While the above is a summary given by Saint Paul, this occurs in a number of instances in Christianity, the most well known of which is that of Matthew 22:36-40 (the following from NKJV)

"Teacher, which is the great commandment in the law?"

Jesus said to him, "'You shall love the Lord your God with all your heart, with all your soul, and with all your mind.' This is the first and great commandment. And the second is like it: 'You shall love your neighbor as yourself.' On these two commandments hang all the Law and the Prophets."

Other instances of the golden rule in the New Testament:

It should not be said that this is the first occurrence of the Golden Rule in the Bible:

You shall not take vengeance, nor bear any grudge against the children of your people, but you shall love your neighbor as yourself: I am the Lord.
Leviticus 19:18 NKJV
and again
What is hateful to you, do not to your fellowman. That is the entire law; all the rest is commentary.
(Talmud, Shabbat 31a)

Islam builds on this idea again with the statement No one of you is a believer until he desires for his brother that which he desires for himself (Forty Hadith of an-Nawawi 13). Sufism continues upon this again with The basis of Sufism is consideration of the hearts and feelings of others. If you haven't the will to gladden someone's heart, then at least beware lest you hurt someone's heart, for on our path, no sin exists but this. (Dr. Javad Nurbakhsh, Master of the Nimatullahi Sufi Order).

One cannot even say that this concept of the Golden Rule is the exclusive property of western spiritual traditions.

this is the sum of duty: Do naught unto others which would cause you pain if done to you. (Mahabharata 5:1517)

One should not behave towards others in a way which is disagreeable to oneself. This is the essence of morality. All other activities are due to selfish desire. (Mahabharata, Anusasana Parva 113.8 )

Hurt not others in ways that you yourself would find hurtful. (Udana Varga 5:18)

...a state that is not pleasing or delightful to me, how could I inflict that upon another? (Samyutta NIkaya v. 353)

Tzu-kung asked, Is there a single word which can be a guide to conduct throughout one's life? The Master said, It is perhaps the word shu. Do not impose on others what you yourself do not desire (Analects 15:23-24 / Doctrine of the Mean 13.3)

Try your best to treat others as you would wish to be treated yourself, and you will find that this is the shortest way to benevolence. (Mencius VII.A.4)

Regard your neighbor's gain as your own gain and your neighbor's loss as your own loss. (T'ai Shang Kan Ying P'ien)

I am good to the man who is good to me, likewise, I am also good to the bad man. (Tao Te Ching)

That nature alone is good which refrains from doing unto another whatsoever is not good for itself. (Dadistan-I-dinik 94:5)

Whatever is disagreeable to yourself do not do unto others. (Shayast-na-Shayast 13:29)

Ascribe not to any soul that which thou wouldst not have ascribed to thee, and say not that which thou doest not." "Blessed is he who preferreth his brother before himself." (Baha'u'llah)

He should not wish for others that which he doth not wish for himself, nor promise that which he doth not fulfill. (The Book of Certitude)

And if thine eyes be turned towards justice, choose thou for thy neighbor that which thou choosest for thyself. (Epistle to the Son of the Wolf, 30)

African and Native American
One going to take a pointed stick to pinch a baby bird should first try it on himself to feel how it hurts. (African Traditional Religions. Yoruba Proverb (Nigeria))

Respect for all life is the foundation. (Six Nations Iroquois Confederacy - The Great Law of Peace)

The heart of the person before you is a mirror. See there your own form
Philsophers (and some who are warily given the title of philsopher) have echoed this basic law in their own teachings
  • Do not do unto others what angers you if done to you by others. (Isocrates 4th century BCE)
  • May I do to others as I would that they should do unto me. (Plato 4th century BCE)
  • Do not do to others that which would anger you if others did it to you. (Socrates 5th century BCE)
  • Treat your inferiors as you would be treated by your superiors, (Secena; Epistle 47:11 1st century CE)
  • What you shun enduring yourself, attempt not to impose on others. You shun slavery-- beware of enslaving others! If you can endure to do that, one would thing you had been once upon a time a slave yourself. For Vice has nothing in common with virtue, nor Freedom with slavery. (Epictetus ; Golden Sayings XLI 1st century CE)
  • 20: Try to treat others as you would want them to treat you. (L. Ron Hubbard; The Way to Happiness 20th century CE)

Some theologians try to say one rule is better than the other - that one rule is more "positive" than the other. This may be argued until you are blue in the face. The essence is that at the core of almost every every religion is the idea of the Golden Rule.

For society to function - for groups of people to work together this most basic of social contracts had to be established. The Romans said it as The law imprinted on the hearts of all men is to love the members of society as themselves. This is often called the Ethics of Reciprocity. While specific items are often iterated in religions to reinforce the ideas or emphasize one aspect of the Golden Rule over another, it is the Golden Rule that runs as a theme throughout religion. This is why it seems as if there are only a few basic laws or commandments across cultures - because in actuality, there is only one.

Every religion emphasizes human improvement, love, respect for others, sharing other people's suffering. On these lines every religion had more or less the same viewpoint and the same goal. (The Dalai Lama)