shri bhagavan uvaca –
bahuni me vyatitani janmani tava carjuna
tany-aham veda sarvani na tvam vettha parantapa

Bhagavan Sri Krishna said: O conqueror of the enemy, both you and I have passed through many births. I know all of them but you do not.

ajo’pi sannavyayatma bhutanam-ishvaro’pi san
prakritim svam-adhishthaya sambhavamy-atmamayaya

You should know that although I am the creator, controller and master of all that be and I appear to take birth, I am not actually born. I appear in this universe in every millennium in My original spiritual form and that form never deteriorates.

yada yada hi dharmasya glanir bhavati bharata
abhyutthanam adharmasya tadatmanam srijamyaham

O descendent of Bharata, whenever there is a decline in dharma and a rise of adharma, at that time I personally appear.

paritranaya sadhunam vinashaya ca dushkritam
dharma-samsthapanarthaya sambhavami yuge yuge

To protect the pious living beings and to put an end to malevolence, I appear in every age to establish dharma.

janma karma ca me divyam evam yo vetti tattvatah
tyaktva deham punar-janma naiti mameti so’rjuna

One who understands My divine appearance and activities never takes birth again after giving up this material body. He comes to Me, O Arjuna.

To understand Sri Krishna’s appearance and activities is indeed to become situated beyond the cycle of birth and death. In material life all living beings are in a perpetual state of transmigration from one birth to the next. Only when one attains pure spiritual consciousness does this samsara, or transmigration, cease. Krishna tells Arjuna that both of them have passed through many births that Arjuna has forgotten, but Krishna remembers them all.
Because the living beings change bodies at the time of death, they also forget their previous lives. Krishna is the Absolute Truth and thus He does not change His body or transmigrate to another body at any time. Because He does not undergo a change of body He does not forget. Krishna is non-different from His body, whereas living beings in material life are units of consciousness that are embodied by material elements. The bodies of all living beings in the material world are made of the basic elements of earth, water, fire, air, space, mind, intelligence and false ego. Krishna is sat-chid-ananda – eternity, knowledge and bliss. Krishna’s body is also sat-chid-ananda, thus Krishna and Krishna’s body are non-different – transcendental spiritual substances.
Sri Krishna not only remembers all His previous births, but He remembers all of Arjuna’s previous births also. This is the characteristic of the Absolute Truth who is fully omniscient.
The knowledge of yoga being lost naturally results in a decline in dharma and a rise of adharma (false dharma). Malevolence arises out of adharma. When this occurs, Krishna says that He appears in the world to re-establish the principles of dharma. Dharma is understood as duties, activities and practices that will sustain the living beings in a state of prosperity and enable them to realize their constitutional position as conscious parts and parcels of the Absolute Truth, Krishna. As such, dharma should not be confused with the mundane religions of this world.
In verse eight Krishna says that He appears in every age (yuge yuge) to establish the yuga-dharma. In Satya-yuga, Krishna appeared as Hamsa, Matsya, Kurma, Varaha and Narasimha avataras. In Treta-yuga, He appeared as Vamana, Parashurama and Ramachandra avataras. In Dvapara-yuga, He appeared as Sri Krishna and in Kali-yuga, He has appeared as Buddha and Sri Chaitanya Mahaprabhu. There is one more avatara yet to appear at the end of Kali-yuga, some 427,000 years from now, and that is Kalki-avatara.
When Krishna was speaking Bhagavad-gita, it was at the end of Dvapara-yuga – an age of considerable piety where open degradation such as clubs and establishments for the consumption of alcohol, illicit sex, political corruption, drug abuse and the organized slaughter of animals were completely unheard of. Now, five thousand years on, we are in the midst of the age known as Kali-yuga where the unheard of vices in Dvapara-yuga are the norm of the day.
Similarly, as Krishna had appeared at the end of Dvapara-yuga, He again appeared after the first 4576 years of Kali-yuga had passed as the avatara, Sri Chaitanya Mahaprabhu, also known as the Kali-yuga avatara, or the yugavatara. As the yugavatara, Krishna taught the dharma of nama-sankirtana, the chanting of the maha-mantra as not only the most important process of self-realization, but as the only recommended process of self-realization in Kali-yuga. Conclusively, the Brihan-Naradiya Purana says:

harer-nama harer nama harer nama eva kevalam
kalau nasty eva nastya eva nasty eva gatir anyatha

In the age of Kali there is no other way, there is no other way, there is no other way except for the chanting of the names of Hari (Krishna).

When the maha-mantra is chanted congregationally in a loud tone it is called kirtana or sankirtana. When the maha-mantra is chanted softly and the repetition is kept count of on a string of one hundred-and eight beads it is called japa.

Since the advent of Sri Chaitanya Mahaprabhu and the sankirtana movement, many great and learned scholars, philosophers and yogis such as Sarvabhauma Bhattacharya and Prakashananda Sarasvati have put aside other systems of yoga, Vedanta and philosophy in favor of becoming fully absorbed in the chanting of the holy names of Krishna. According to great self-realized personalities, the chanting of the maha-mantra is the surest path to spiritual perfection in this age. Srimad Bhagavatam states as follows:

kaler dosha nidhe rajan asti hy eko mahan gunah
kirtanad eva krishnasya mukta-sangah param vrajet

Although Kali-yuga is an ocean of faults where people are short-lived, slow in self-realization and always disturbed, still there is one great quality about this age – simply by chanting the name of Krishna, one can be delivered from material bondage and attain the supreme destination. (Srimad Bhagavatam 12.3.51)

krite yad dhyayato vishnum tretayam yajato makhaih
dvapare paricharyayam kalau taddhari-kirtanat

Whatever results were gained in Satya-yuga by meditating upon Vishnu, in Treta-yuga by performing elaborate sacrifices and in Dvapara-yuga by Deity worship can be obtained in Kali-yuga simply by chanting the names of Krishna. (Srimad Bhagavatam 12.3.52)

The chanting of the maha-mantra advances one in self-realization because it purifies the heart of material influences and eliminates the false conceptions of life, thus terminating the cycle of birth and death. Sri Chaitanya Mahaprabhu has composed a verse wherein the benefits of sankirtana, chanting the maha-mantra, have been described as follows:

cheto-darpana-marjanam bhava maha-davagni nirvapanam
sreyah-kairava-chandrika-vitaranam vidya-vadhu-jivanam
anandambudhi-vardhanam prati-padam purnamritasvadanam
sarvatma-snapanam param vijayate sri krishna-sankirtanam

The holy name of Krishna cleanses the mirror of the heart and extinguishes the fire of misery in the forest of birth and death. Just as the evening lotus blooms in the moons cooling rays, the heart begins to blossom in the nectar of Krishna’s name. And at last the atma awakens to its real inner treasure – a life of love with Krishna. Again and again tasting nectar, the atma dives and surfaces in the ever-increasing ocean of ecstatic joy. All phases of the self of which we may conceive are fully satisfied and purified, and at last conquered by the all-auspicious influence of the holy name of Krishna. (Shikshashtaka 1)

Sri Chaitanya Mahaprabhu taught the chanting of the maha-mantra and a complete system of philosophy known as achintya-bhedabheda-tattva that has encompassed all the great philosophical systems of India that preceded Him, such as Shankara’s advaita, Vishnu Svami’s shuddhadvaita, Nimbarka’s dvaitadvaita, Ramanuja’s vishishthadvaita and Madhva’s dvaita. The achintya-bhedabheda-tattva philosophy is essentially the philosophy of simultaneous oneness and difference in the Absolute Truth, culminating in prema-bhakti or divine love. As such, Sri Chaitanya Mahaprabhu has revealed the greatest philosophy of spiritual perfection in this world.
To accompany the chanting of the maha-mantra the process of Deity worship that was prominent in Dvapara-yuga is still in vogue today. The Deity is the archa-vigraha representation of Sri Krishna that is manifest before the aspirant so that one can perform archana (worship) and fix the mind and senses on the form of the Supreme Person. When the authorized archa-vigraha is present, such worship should not be confused with the worship of lifeless and unauthorized idols. Current in the communities of bhakti-yoga are the worship of the archa-vigrahas of Sri Krishna such as Jagannatha, Pancha-tattva, Gaura-Nitai, Gaura-Gadadhara, Sri Narasimha and Sri Sri Radha-Krishna.