Does it matter what mantra I chant?
by His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
"The guru does not manufacture a new process to instruct the disciple. The disciple receives from the guru an authorized process received by the guru from his guru. This is called the system of disciplic succession (evaḿ paramparā-prāptaḿ imaḿ rājarṣayo viduḥ [Bg. 4.2]). This is the bona fide Vedic system of receiving the process of devotional service, by which the Supreme Personality of Godhead is pleased.
Therefore, to approach a bona fide guru, or spiritual master, is essential. The bona fide spiritual master is he who has received the mercy of his guru, who in turn is bona fide because he has received the mercy of his guru. This is called the parampara system.
Unless one follows this parampara system, the mantra one receives will be chanted for no purpose. Nowadays there are so many rascal gurus who manufacture their mantras as a process for material advancement, not spiritual advancement. Still, the mantra cannot be successful if it is manufactured.
Mantras and the process of devotional service have special power, provided they are received from the authorized person."
- from the Srimad-Bhagavatam (8.16.24, purport)