Sanatana Dharma also known as Sanaatan Dharma is a sanskrit term meaning "eternal law," "eternal religion" or "the eternal unchanging occupation of the soul in its relation to the Supreme Being." Thus one who follows the principles of Sanatana Dharma is led back to a pure state of consciousness of a unified relationship with the Source.
Many make the mistake of equating Hinduism and Sanatana Dharma as the same concept which isn't necessarily true . First, one must understand that the term "Hindu" or "Hinduism" isn't even a Sanskrit name. It isn't found in any of the Vedic literature. The term "hindu" doesn't seem to appear until around 325 B.C. One account states that Alexander the Great upon invading the area now known as India first began calling the Sindhu River simply as the Indus deleting the "S" supposedly because it was easier for Greeks to pronounce. His Macedonian armies thereafter called the land east of the Indus as India which became the norm during the British regime.
Later as Muslim invaders came to the area they began calling it "Hindu" and the inhabitants of the area as Hindus. This is supposedly because the Sanskrit sound of "S" converts to "H" in the Parsee language. Eventually the Indians began to conform to these standards and used the names Hindu and Hindustan. Of course this led to greater confusion when the word was also used to identify the religion of the inhabitants which became "Hinduism." This became a tem of convenience for the British to identify the Muslims in the area from the Hindus in the area. It is said that this also served the double benefit of keeping the Muslims and Hindus at odds and therefore allowing for continued British control of the area through means of "divide and rule."
Thus those that state "Hinduism" is the oldest religion may be correct and yet it is somewhat of a misnomer as well. What they are actually attempting to convey is that the "spiritual practices" based upon the Vedas, Upanishads, Bhagavad Gita and Puranas are the oldest religious systems known. This may be the closest synonym to Santana Dharma but in fact the "eternal law" transcends even the Vedas for prior to even the Vedas being written down the laws of the relationship between soul and Spirit were ever present.
Sanatana Dharma represents this "Universal Truth" that all of creation even down to the atom is merely a conscious projection of the Creator and is ultimately destined to merge back consciously into the Supreme Spirit. It is said that in the beginning of creation the soul is an eternal aspect of Spirit. The best analogy may be that of the relationship between the sun and the sun-ray. As long as the sun shines, the sun-ray is an intrinsic part of it and possesses all the qualities of its source. The further the sun ray travels from its source the "less potent" it becomes. Because the Source is aware of this characteristic, It makes for a way for that sun ray to always return back to its True Essence - this is Sanatana Dharma! Therefore, Sanatana Dharma doesn't belong to any one religion. It is a universal path applied by all people, in any part of the world, in any time in history and in any religion.
Being an eternal spiritual truth, Sanatana Dharma is beyond all time and worldly designations. All religions may come and go but Sanatana Dharma or the principles of Sanatana Dharma shall forever remain. Thus every spiritual expression of Truth who has ever walked the face of the earth has taught the principles of Sanatana Dharma. They never came to start a religion or to preach a new truth. Jesus, Krishna, Buddha, Mohammed or any other saint understood the eternal nature of Spirit. It is only through their varied individual expressions of this One Truth that led to the many forms of religion such as Judaism, Christianity, Islam, Hinduism, Buddhism, etc.
Sanatana Dharma is not a man-made religion but rather how God brings all things into manifestation and how every soul, each a "prodigal son" can return and merge back into Source. It is said Sanatana Dharma isn't a revelation in and of itself but more so the "fruit" of revelation. It shows people how to achieve for themselves the Highest Truth which may often manifest Itself as Infinite Love, Peace, Light, Sound, Bliss and so forth. St. Paul described it as "the peace of God, which passeth all understanding." Yogis describe it as Samadhi - "spiritual ecstacy!" Buddha called it "Nirvana" - the perfect state of being free from all suffering. And St. Teresa of Avila described it as "the formless Christ."
It is only when individuals who have not experienced the state of Oneness that people become at odds over semantics. If we were to try and describe fire we would get all sorts of answers. Some might say its red, orange or yellow... unless of course it's white hot like a burning star. If you get too close you may say it's hot unless of course it is cold outside then you may say its warm. If you get burned by it then you may say it is painful but if it helps you to see your way then you will say it is benevolent. Fire is still fire it just depends upon your experience of it and your own mental perceptions. Either way words are always limiting in attempting to define an "experience" or "state of being."
Sanatana Dharma emphasizes the need for inner, direct communion. The Vedas refer to this state as Kavailya meaning complete soul-liberation and eternal union with God. Jesus described it as "the kingdom of God within you!" Thus, Sanantana Dharma itself is the same everywhere within the created universe. It concerns the path which all manifested beings must take who desire eventual reunion with their Divine Source. In truth, there is nothing but Spirit in actuality. Everything in creation is a mere transitory appearance of the Changeless Spirit.
Sri Ramahkrishna described Sanatana Dharma using the imagery of sweetness to explain the Eternal Truth. He says honey, sugar, chocolate, sweetmeats and candies may all vary in form but they contain the same essence in nature - sweetness! Similarly, all religions, religious practices, rituals, prayers, yoga and spirituality all capture some element of Sanatana Dharma to greater or lesser degrees... this is the Eternal Truth! Only the person who transcends all relativities and dualities and comparisons will discover the indescribable state of Absoluteness.