When goodness grows weak,
When evil increases,
I make myself a body.
In every age I come back
To deliver the holy,
To destroy the sin of the sinner
To establish righteousness.
~ Bhagavad Gita ~
An Avatar is in incarnation of a fragment of God on Earth. Avatars that are known of in the West include, Christ, Buddha, Rama, and Krishna; but there are many others as enumerated in the Bhagavata, the story of all the major Avatars. An Avatar may be born in the home of a very pure but otherwise ordinary human mother and father. The Avatar arrives in a body that is human and mortal like others but the Avatar himself is fully aware of his complete Divinity and whatever he wills must happen. The Mahabharata and Bhagavata contain stories about the Avatar Krishna who was recognized by a few of his contemporaries as a Divine Incarnation. But most considered him their friend, enemy, or just another, if extraordinary, citizen of the community.
The Avatar is born with a mission, a reason for his advent and never swerves from those goals. Avatars may accomplish tasks in miraculous ways or they may use ordinary means and instruments to accomplish their goal. The miracles, however are merely the calling card of the Divine and not the end in themselves. Those few pure souls, full of devotion, who are drawn to the Avatar may glimpse a portion of his glory and think of Him as a great saint, sage, or more rarely even recognize Him for what he is, an incarnation of the Absolute, but they are typically few in number. Powers of evil naturally oppose the powers of good, so often men of power and arrogance develop a strong hatred of divine personalities and oppose them vigorously as was the case with Christ.
God has no bondage to time and space. For him, all beings are the same. He is the master of the living and the non-living. At the conclusion of every aeon the process of involution is completed in the Deluge; then, evolution starts again and as Brahma (the Creator), He creates beings again. He enlightens every one with a spark of His own Glory and fosters on the path of fulfillment every one of them, as Vishnu (the Preserver). It is He again, who as Shiva (the Destroyer), concludes the process by the destruction of all. Thus you can see that there is no limit to His Might, no end to his Potence.
There can be no boundaries for His achievements. He incarnates in countless ways; He comes as an Incarnation of a Kala (fragment) of His, or an Amsa (part) of His; He comes as an Inner inspirer for some definite Purpose; He comes to close an epoch and inaugurate another (Yugavathar). The narrative of these Incarnations is the Bhagavatha.
The power that various Avatars wield differs greatly. God sometimes incarnates with full powers and sometimes a smaller portion of His glory depending on His purpose. Full Avatars are born only in India, the ancient land of Bharath, because only in India have the Vedas and Sastras been understood and practiced by sages over the milleniums. Partial Avatars may be born anywhere on Earth. Another type of Avatar is called a "Messenger of God". Such a one was Christ. Sages and Saints are quite distinct from Avatars, being pure human souls aspiring to God. The difference between an Avatar and a sage or saint is this: the full Avatar is enlightened and completely aware of his own Divinity even at birth. The Avatar is never overcome by the maya (illusion) of the world, and is always aware of His mission and the Divine powers at His command. In contrast a sage or saint is a human who has attained some degree of spiritual progress and is reborn in ever higher states until, in a final life, that sage or saint attains to Self-Realization or mergence in the Absolute. The sage or saint is devoted to God and practices meditation or rituals or prayers, depending on the path, as spiritual sadhana to approach ever closer to the Supreme Being.
Some spiritual beings, even after attaining to Realization may choose to be reborn voluntarily to further some specific plan or purpose of the Divine. There are many examples of such types of beings in India and elsewhere who have come to accomplish various tasks. Many times their activities are carried out using very subtle means. In the East such a one is called a Sat Purush. Various religions use other terms such as a Saint, Prophet, Buddha, etc.
Once one is established in infinite consciousness,
one becomes silent and though knowing everything,
goes about as if he does not know anything
Though he might be doing a lot of things in several places,
to all outward appearances, he will remain as if he does nothing.
~ Nityananda ~
Sometimes, when the created world has to be sustained, I myself assume name and form and initiate Manvantharas (great eras of time) and provide the Earth with appropriate Divine Personalities and Sages, who set examples to be followed and indicate the path of progress.
I end also the unlimited increase of beings, when it happens. For this sake, I take on the form of Rudra (the destructive aspect of God). I create the bad, in order to high-light and promote the good; and in order to protect the good, I set certain limits, both to the good and the bad, for, they would otherwise, stray into wrong ways and inflict great harm. (Bhagavata)
Western theologians argue about whether God is transcendent or immanent. Does he create the universe and then withdraw, leaving it to run under it's own momentum or does he remain in attendance, overseeing it's operation? The answer, according to Vedanta, is that God is both transcendent and imminent.
The eternal Supreme Being spins out the universe and pervades all beings as the inner witness (and enjoyer of all), and also pervades all the matter in the universe, yet He remains unchanged as the unmanifest support behind the manifested universe. There is one other special feature of this scheme of creation, and that is that God himself takes form within the various spheres and levels of the universe to both bless and guide the beings within them. We know from our own experience that no process can continue unchanged but must be constantly tended to maintain itself. If we plant a garden for example, we must return periodically to water and fertilize, cultivate and weed. Similarly God incarnates as needed to foster good and suppress evil. But that is not the only reason for the descent of an Avatar.
As stated in the Bhagavatha Vahini text, as penned by Sathya Sai Baba:
People believe that incarnations of God happen only for two reasons: the punishment of the wicked and the protection of the righteous. But, these represent only one aspect of the Task. The granting of peace and joy, of a sense of fulfillment to seekers who have striven long - this too is the Task.
The Avatar or Form Incarnate is only the concretisation of the yearning of the seekers. It is the solidified sweetness of the devotion of godly aspirants. The formless assumes the Form for the sake of these aspirants and seekers.
There have been many instances of partial Avatars such as Veda Vyasa, who codified the Vedas and Shankaracharya who purified the teachings and gave renewed strength to the nondualistic philosophy of Advaitha. Partial incarnations of divine personalities happen in all types of cultures and all historic times but the full incarnation is a very infrequent event:
There is a third category of personages or august or exalted beings, who, though embodied in a human frame, in truth are super beings. From time immemorial such personages, albeit only on extremely rare occasions, take birth on this earth, live, and work amongst the common humanity and guide an immense number of them to the Supreme Wisdom. They are the highest manifestations of the Primary Consciousness or Almighty God and therefore from the very moment of their birth they have the Absolute Wisdom about the nature of the whole cosmos and all its contents. They also have Absolute knowledge of Primary Consciousness, in other words, the knowledge about their own Divinity and Godhead. These super beings are called Purna Avatars (also Poorna or Full), or Supreme Incarnations of Almighty God, in Sanathana Dharma (Ancient Wisdom) or the Perennial Religion. Purna Avatars are born with an extraordinary array of supernatural powers, and possess an inexhaustible ability to perform divine miracles or miraculous deeds. They use these supernatural powers of theirs with the highest motives for both spiritual and temporal good of other beings, especially human beings. They never use these powers to benefit themselves.
Up to now, in the whole history of mankind, all together four such Purna Avatars or Supreme Incarnations of Almighty God have taken birth on earth.
From Adwaita Rahasya - Secrets of Creation Revealed by Dr. Chandra Bhan Gupta
When He comes down assuming special form on special occasions for a specific purpose, He is known as Avathara. In fact, Manu and Prajapati and other Persons are Divine Persons entrusted by Brahma (the Creator aspect) with the mission of peopling the world. Everything happens in consonance with the Divine Will. So we can assert that the saints, sages, ascetics, and men both good and bad are all Avatharas of the Vishnu (Preserver) Entity (also called Hari). Avatharas are as countless as living beings are, for, each is born as a consequence of Divine Will. But the story of the Yugavathar (The Avatar of the Age) alone is worth perusal for the Advent is to restore Dharma (righteousness) and moral life.
In a strange way we are all Avatars since we all contain the inner Atma. But since we identify with our limited individuality and human powers, we are Avatars only as a potentiality. A true Avatar by contrast is always aware that he is God in the form of a human and has the full power of the universe and beyond behind him.