What exactly is meant by Harernamaiva Kevalam
Narada Muni was repeating to many other similar verses found in other Mahapuranas i.e., Vishnu Purana and Bhagavatam in different matters.
The mode of worship change from time to time and yuga to yuga due to change in conditions and the ability of the worshippers. The spiritual levels of people has also fallen. We do not have much time to perform penances or do meditation. We even have problems in understanding the esoteric meaning of the Vedas and other holy scriptures. If much time is put in that, when do we apply those teachings and strive for moksha? Hence, the provision was made. This is the easiest way.
Vyasa says the following in Amsha 6 adhyaya 2 of Vishnu Purana
The fruit of penance, of continence, of silent prayer, and the like, practised in the Krita age for ten years, in the Treta for one year, in the Dwápara for a month, is obtained in the Kali age in a day and night: therefore did I exclaim "Excellent, excellent, is the Kali age!" That reward which a man obtains in the Krita by abstract meditation, in the Treta by sacrifice, in the Dwápara by adoration, he receives in the Kali by merely reciting the name of Keshava.
kaliṁ sabhājayanty āryā guṇa jñāḥ sāra-bhāginaḥ
yatra saṅkīrtanenaiva sarva-svārtho ’bhilabhyate (Srimad Bhagavatam Skandha 11 chapter 5 shloka 36)
Those who are actually advanced in knowledge are able to appreciate the essential value of this Age of Kali. Such enlightened persons worship Kali-yuga because in this fallen age all perfection of life can easily be achieved by the performance of saṅkīrtana.
There is also a statement in the Uttara Khanda of Ramayana where the advisors of Sri Rama says the characteristics of Yuga change and the worshipping methods also change. Hence the spiritual level of all the Varnas will be the same i.e., a Brahmana will be the same as Shudra (I have to find exact verse though).
More instances were added in an answer written by Keshav Srinivasan Do methods of worship change according to yugas?
Chaitanya Mahaprabhu emphasized on Sankirtana out of nava vidha bhaktis. Hence he said that Hari Nama sankirtana is enough for getting liberation.
Adi Shankaracharya also says in his Bhaja Govindam stotra :
govindaM bhajamuuDhamate .
saMpraapte sannihite kaale
nahi nahi rakshati DukRiJNkaraNe
Worship Govinda, Worship Govinda, Worship Govinda. Oh fool! Rules of Grammar will not save you at the time of your death.
bhagavad.h giitaa kiJNchidadhiitaa
gaNgaa jalalava kaNikaapiitaa .
sakRidapi yena muraari samarchaa
kriyate tasya yamena na charchaa
Let a man read but a little from Gitaa, drink just a drop of water from the ganges, worship murari (govinda) just once. He then will have no altercation with Yama. Stanza attributed to dR^iDhabhakta.
In the above verse, Adi Shankaracharya recommends worship of Krishna just even once and says the person will have no altercation with Yama which means he will not have to visit Naraka (meaning Lord will take care and give better lokas).
So, the verse strongly suggests the nama sankirtana of Lord Vishnu. That doesn't mean we should leave our duties of performing Sandhyavandanam and deeds like visiting temples. Lord Krishna states one should do his own duty. The verse is suggesting that Lord Vishnu's name is enough for liberation.
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