Srimad Bhagavatam Canto 09, Chapter 14, Text 48

SB 9.14.48

eka eva pura vedah
 pranavah sarva-vanmayah
devo narayano nanya
 eko ’gnir varna eva ca
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
In the Satya-yuga, the first millennium, all the Vedic mantras were included in one mantra — pranava, the root of all Vedic mantras. In other words, the Atharva Veda alone was the source of all Vedic knowledge. The Supreme Personality of Godhead Narayana was the only worshipable Deity; there was no recommendation for worship of the demigods. Fire was one only, and the only order of life in human society was known as hamsa.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
In Satya-yuga there was only one Veda, not four. Later, before the beginning of Kali-yuga, this one Veda, the Atharva Veda (or, some say, the Yajur Veda), was divided into four — Sama, Yajur, Rg and Atharva — for the facility of human society. In Satya-yuga the only mantra was omkara (om tat sat). The same name omkara is manifest in the mantra Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare. Unless one is a brahmana, one cannot utter omkara and get the desired result. But in Kali-yuga almost everyone is a sudra, unfit for pronouncing the pranava, omkara. Therefore the sastras have recommended the chanting of the Hare Krsna maha-mantra. Omkara is a mantra, or maha-mantra, and Hare Krsna is also a maha-mantra. The purpose of pronouncing omkara is to address the Supreme Personality of Godhead, Vasudeva (om namo bhagavate vasudevaya). And the purpose of chanting the Hare Krsna mantra is the same. Hare: “O energy of the Lord!” Krsna: “O Lord Krsna!” Hare: “O energy of the Lord!” Rama: “O Supreme Lord, O supreme enjoyer!” The only worshipable Lord is Hari, who is the goal of the Vedas (vedais ca sarvair aham eva vedyah). By worshiping the demigods, one worships the different parts of the Lord, just as one might water the branches and twigs of a tree. But worshiping Narayana, the all-inclusive Supreme Personality of Godhead, is like pouring water on the root of the tree, thus supplying water to the trunk, branches, twigs, leaves and so on. In Satya-yuga people knew how to fulfill the necessities of life simply by worshiping Narayana, the Supreme Personality of Godhead. The same purpose can be served in this Age of Kali by the chanting of the Hare Krsna mantra, as recommended in the Bhagavatam. Kirtanad eva krsnasya mukta-sangah param vrajet. Simply by chanting the Hare Krsna mantra, one becomes free from the bondage of material existence and thus becomes eligible to return home, back to Godhead.
Srimad Bhagavatam Canto 09, Chapter 14, Text 47
Srimad Bhagavatam Canto 09, Chapter 14, Text 49