Valmiki Maharshi the Adikavi himself was a proof of the efficacy of Ramanama in purifying human souls.
He was originally Ratnakara, a forest-hunter robbing travellers to earn his livelihood. Narada awakened his conscience by making him understand that his kith and kin who partook of his wealth earned by looting, would not take any share of his sins. Narada Muni initiated him into Ramanama saying that the Taraka Mantra would save him. The illiterate Ratnakara could not even pronounce the letters RAMA RAMA. All that he could utter was MARA MARA. Narada Maharshi said "so be it".
The repetition of MARA MARA MARA gave Ratnakara the effect of chanting Ramanama as Narada rightly guessed.
Retiring to secluded place, Ratnakara chanted the name continuously with all sincerity.
The power of the name and the genuineness of the former decoit's change of heart wrought a miracle. Ratnakara turned his eyes inward and became and 'Aavrutta Chkshu.' His spirit soared to unalloyed bliss. He lost his body consciousness and clung to the divine name as his only support. An anthill formed around him. Hence the name Valmiki.
Legends say that Ratnakara sat there chanting Ramanama for 12000 years. Every cell in his body got attuned to the glorious name of Rama. Ratnakara was transformed into Maharshi Valmiki. The Valmiki episode marks the redeeming power of God, leading even a worst sinner to total transformation and ultimate redemption.