That which delivers one from mental speculation is called "Mantra''. "Mantra Siddhi'' is to transcend the gross and subtle mental plane... In this age all the Mantras that can help us reaching perfection up to the plane of Godhead has been still more concentrated into the Hari-nama. We find therefore in the Brhannaradiya Puranam (38/126) a particular stress on Harinama which is stated as follows:
harinama harinama harinama eva kevalam
kalau nastyeva nastyeva nastyeva gatir anyatha
Letter to Sri Padampat Singhania, 7 May 1957