SB 4.17.5
sanat-kumarad bhagavato
brahman brahma-vid-uttamat
labdhva jñanam sa-vijñanam
rajarsih kam gatim gatah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The great saintly King, Maharaja Prthu, received knowledge from Sanat-kumara, who was the greatest Vedic scholar. After receiving knowledge to be applied practically in his life, how did the saintly King attain his desired destination?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
There are four Vaisnava sampradayas (systems) of disciplic succession. One sampradaya comes from Lord Brahma, one from the goddess of fortune, one from the Kumaras, headed by Sanat-kumara, and one from Lord Siva. These four systems of disciplic succession are still going on. As King Prthu has illustrated, one who is serious about receiving transcendental Vedic knowledge must accept a guru, or spiritual master, in one of these four disciplic successions. It is said that unless one accepts a mantra from one of these sampradayas, the so-called mantra will not act in Kali-yuga. Many sampradayas have sprung up without authority, and they are misleading the people by giving unauthorized mantras. The rascals of these so-called sampradayas do not observe the Vedic rules and regulations. Although they are addicted to all kinds of sinful activities, they still offer the people mantras and thus mislead them. Intelligent persons, however, know that such mantras will never be successful, and as such they never patronize such upstart spiritual groups. People should be very careful of these nonsensical sampradayas. To get some facility for sense gratification, unfortunate people in this age receive mantras from these so-called sampradayas. Prthu Maharaja, however, showed by his example that one should receive knowledge from a bona fide sampradaya. Therefore Maharaja Prthu accepted Sanat-kumara as his spiritual master.