SB 2.7.5
taptam tapo vividha-loka-sisrksaya me
adau sanat sva-tapasah sa catuh-sano ’bhut
prak-kalpa-samplava-vinastam ihatma-tattvam
samyag jagada munayo yad acaksatatman
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
To create different planetary systems I had to undergo austerities and penance, and the Lord, thus being pleased with me, incarnated in four sanas [Sanaka, Sanatkumara, Sanandana and Sanatana]. In the previous creation the spiritual truth was devastated, but the four sanas explained it so nicely that the truth at once became clearly perceived by the sages.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Visnu-sahasra-nama prayers mention the Lord’s name as sanat and sanatana-tama. The Lord and the living entities are both qualitatively sanatana, or eternal, but the Lord is sanatana-tama or the eternal in the superlative degree. The living entities are positively sanatana, but not superlatively, because the living entities are apt to fall to the atmosphere of noneternity. Therefore, the living entities are quantitatively different from the superlative sanatana, the Lord.
The word san is also used in the sense of charity; therefore when everything is given up in charity unto the Lord, the Lord reciprocates by giving Himself unto the devotee. This is also confirmed in the Bhagavad-gita (4.11): ye yatha mam prapadyante. Brahmaji wanted to create the whole cosmic situation as it was in the previous millennium, and because, in the last devastation, knowledge of the Absolute Truth was altogether erased from the universe, he desired that the same knowledge again be renovated; otherwise there would be no meaning in the creation. Because transcendental knowledge is a prime necessity, the ever-conditioned souls are given a chance for liberation in every millennium of creation. This mission of Brahmaji was fulfilled by the grace of the Lord when the four sanas, namely Sanaka, Sanatkumara, Sanandana and Sanatana, appeared as his four sons. These four sanas were incarnations of the knowledge of the Supreme Lord, and as such they explained transcendental knowledge so explicitly that all the sages could at once assimilate this knowledge without the least difficulty. By following in the footsteps of the four Kumaras, one can at once see the Supreme Personality of Godhead within oneself.