The sixteen word Maha Mantra is nobody's invention. There are authorized mantras in the different Puranas which are Vedic authorities. Especially in the Kali Santara Upanisad it is stated that this sixteen word Maha Mantra is the Holy Name of the Supreme Brahma for the deliverance of the fallen souls. Caitanya Mahaprabhu chanted, Haridasa Thakura chanted 24 hours, all the Goswamis and Acaryas chanted, my Guru Maharaja chanted, and following all these footsteps we are also chanting. So they are authorized by disciplic succession, and all of us must chant at least 16 rounds daily to keep us fit on the Krishna Consciousness transcendental platform.
Letter to Ranadhira, 24 January, 1970