SB 8.5.23

ayaṁ ca tasya sthiti-pālana-kṣaṇaḥ
 sattvaṁ juṣāṇasya bhavāya dehinām
tasmād vrajāmaḥ śaraṇaṁ jagad-guruṁ
 svānāṁ sa no dhāsyati śaṁ sura-priyaḥ
Synonyms: 
ayam — this period; ca — also; tasya — of the Supreme Personality of Godhead; sthiti-pālana-kṣaṇaḥ — the time for maintenance, or for establishing His rule; sattvam — the mode of goodness; juṣāṇasya — accepting (now, without waiting); bhavāya — for the increased development or establishment; dehinām — of all living entities who accept material bodies; tasmāt — therefore; vrajāmaḥ — let us take; śaraṇam — shelter; jagat-gurum — at the lotus feet of the Supreme Personality of Godhead, who is the universal teacher; svānām — His own persons; saḥ — He (the Supreme Personality of Godhead); naḥ — unto us; dhāsyati — will give; śam — the good fortune we need; sura-priyaḥ — because He is naturally very dear to the devotees.
Translation: 
Now is the time to invoke the mode of goodness of the living entities who have accepted material bodies. The mode of goodness is meant to establish the Supreme Lord’s rule, which will maintain the existence of the creation. Therefore, this is the opportune moment to take shelter of the Supreme Personality of Godhead. Because He is naturally very kind and dear to the demigods, He will certainly bestow good fortune upon us.
Purport: 

The material world is conducted by the three modes of nature, namely sattva-guṇa, rajo-guṇa and tamo-guṇa. By rajo-guṇa everything material is created, by sattva-guṇa everything material is maintained properly, and by tamo-guṇa, when the creation is improperly situated, everything is destroyed.

From this verse we can understand the situation of Kali-yuga, through which we are now passing. Just before the beginning of Kali-yuga — or, in other words, at the end of Dvāpara-yuga — Lord Śrī Kṛṣṇa appeared and left His instructions in the form of Bhagavad-gītā, in which He asked all living entities to surrender unto Him. Since the beginning of Kali-yuga, however, people have practically been unable to surrender to the lotus feet of Kṛṣṇa, and therefore, after some five thousand years, Kṛṣṇa came again as Śrī Caitanya Mahāprabhu just to teach the entire world how to surrender unto Him, unto Śrī Kṛṣṇa, and thus be purified.

Surrendering unto the lotus feet of Kṛṣṇa means achieving complete purification. Kṛṣṇa says in Bhagavad-gītā (18.66):

sarva-dharmān parityajya
 mām ekaṁ śaraṇaṁ vraja
ahaṁ tvāṁ sarva-pāpebhyo
 mokṣayiṣyāmi mā śucaḥ

“Abandon all varieties of religion and just surrender unto Me. I shall deliver you from all sinful reaction. Do not fear.” Thus as soon as one surrenders unto the lotus feet of Kṛṣṇa, one certainly becomes free from all contamination.

Kali-yuga is full of contamination. This is described in the Śrīmad-Bhāgavatam (12.3.51):

kaler doṣa-nidhe rājann
 asti hy eko mahān guṇaḥ
kīrtanād eva kṛṣṇasya
 mukta-saṅgaḥ paraṁ vrajet

This Age of Kali is full of unlimited faults. Indeed, it is just like an ocean of faults (doṣa-nidhi). But there is one chance, one opportunity. Kīrtanād eva kṛṣṇasya mukta-saṅgaḥ paraṁ vrajet: simply by chanting the Hare Kṛṣṇa mantra, one can be freed from the contamination of Kali-yuga and, in his original spiritual body, can return home, back to Godhead. This is the opportunity of Kali-yuga.

When Kṛṣṇa appeared, He gave His orders, and when Kṛṣṇa Himself appeared as a devotee, as Śrī Caitanya Mahāprabhu, He showed us the path by which to cross the ocean of Kali-yuga. That is the path of the Hare Kṛṣṇa movement. When Śrī Caitanya Mahāprabhu appeared, He ushered in the era for the saṅkīrtana movement. It is also said that for ten thousand years this era will continue. This means that simply by accepting the saṅkīrtana movement and chanting the Hare Kṛṣṇa mahā-mantra, the fallen souls of this Kali-yuga will be delivered. After the Battle of Kurukṣetra, at which Bhagavad-gītā was spoken, Kali-yuga continues for 432,000 years, of which only 5,000 years have passed. Thus there is still a balance of 427,000 years to come. Of these 427,000 years, the 10,000 years of the saṅkīrtana movement inaugurated by Śrī Caitanya Mahāprabhu 500 years ago provide the opportunity for the fallen souls of Kali-yuga to take to the Kṛṣṇa consciousness movement, chant the Hare Kṛṣṇa mahā-mantra and thus be delivered from the clutches of material existence and return home, back to Godhead.

Chanting of the Hare Kṛṣṇa mahā-mantra is potent always, but it is especially potent in this Age of Kali. Therefore Śukadeva Gosvāmī, while instructing Mahārāja Parīkṣit, stressed this chanting of the Hare Kṛṣṇa mantra.

kaler doṣa-nidhe rājann
 asti hy eko mahān guṇaḥ
kīrtanād eva kṛṣṇasya
 mukta-saṅgaḥ paraṁ vrajet

“My dear King, although Kali-yuga is full of faults, there is still one good quality about this age. It is that simply by chanting the Hare Kṛṣṇa mahā-mantra, one can become free from material bondage and be promoted to the transcendental kingdom.” (Bhāg. 12.3.51) Those who have accepted the task of spreading the Hare Kṛṣṇa mahā-mantra in full Kṛṣṇa consciousness should take this opportunity to deliver people very easily from the clutches of material existence. Our duty, therefore, is to follow the instructions of Śrī Caitanya Mahāprabhu and preach the Kṛṣṇa consciousness movement all over the world very sincerely. This is the best welfare activity for the peace and prosperity of human society.

Śrī Caitanya Mahāprabhu’s movement consists of spreading kṛṣṇa-saṅkīrtana. Paraṁ vijayate śrī-kṛṣṇa-saṅkīrtanam: “All glories to the Śrī Kṛṣṇa saṅkīrtana!” Why is it so glorious? This has also been explained by Śrī Caitanya Mahāprabhu. Ceto-darpaṇa-mārjanam: by the chanting of the Hare Kṛṣṇa mahā-mantra, one’s heart is cleansed. The whole difficulty is that in this Age of Kali there is no sattva-guṇa and no clearance of the heart, and therefore people are making the mistake of identifying with their bodies. Even the big philosophers and scientists with whom we deal are practically all under the impression that they are their bodies. The other day we were discussing a prominent philosopher, Thomas Huxley, who was proud of being an Englishman. This means that he was in the bodily conception of life. Everywhere we find this same misunderstanding. As soon as one is in the bodily conception of life, one is nothing but an animal like a cat or a dog (sa eva go-kharaḥ). Thus the most dangerous of the dirty things within our hearts is this misidentification of the body as the self. Under the influence of this misunderstanding, one thinks, “I am this body. I am an Englishman. I am an Indian. I am an American. I am Hindu. I am Muslim.” This misconception is the strongest impediment, and it must be removed. That is the instruction of Bhagavad-gītā and of Śrī Caitanya Mahāprabhu. Indeed, Bhagavad-gītā begins with this instruction:

dehino ’smin yathā dehe
 kaumāraṁ yauvanaṁ jarā
tathā dehāntara-prāptir
 dhīras tatra na muhyati

“As the embodied soul continually passes, in this body, from boyhood to youth to old age, the soul similarly passes into another body at death. The self-realized soul is not bewildered by such a change.” (Bg. 2.13) Although the soul is within the body, nevertheless, because of misunderstanding and animal propensities one accepts the body as the self. Caitanya Mahāprabhu therefore says, ceto-darpaṇa-mārjanam. To cleanse the core of the heart, which is full of misunderstanding, is possible only through śrī-kṛṣṇa-saṅkīrtana. The leaders of the Kṛṣṇa consciousness movement should very seriously take this opportunity to be kind to the fallen souls by delivering them from the misunderstanding of materialistic life.

One cannot be happy in any way within this material world. As stated in Bhagavad-gītā (8.16):

ābrahma-bhuvanāl lokāḥ
 punar āvartino ’rjuna

“From the highest planet in this material world down to the lowest, all are places of misery wherein repeated birth and death take place.” Therefore, not to speak of going to the moon, even if one is promoted to the highest planetary system, Brahmaloka, there cannot be any happiness in this material world. If one actually wants happiness, one must go to the spiritual world. The material world is characterized by a struggle for existence, and survival of the fittest is a well-known principle, but the poor souls of this material world do not know what is survival and who is fit. Survival does not mean that one should die; survival means that one should not die, but should enjoy an everlastingly blissful life of knowledge. This is survival. The Kṛṣṇa consciousness movement is meant to make every person fit for survival. Indeed, it is meant to stop the struggle for existence. The Śrīmad-Bhāgavatam and Bhagavad-gītā give definite directions on how to stop the struggle for existence and how to survive in eternal life. The saṅkīrtana movement, therefore, is a great opportunity. Simply by hearing Bhagavad-gītā and chanting the Hare Kṛṣṇa mahā-mantra, one becomes completely purified. Thus the struggle for existence ceases, and one goes back home, back to Godhead.