It is most certain that fire burns wood to ashes. But if the log of wood is huge, it calls for more fire in the beginning, and that very fire expands and grows itself to burn the log completely.
Likewise, Mahamantra is capable of destroying one’s Karma whatever they may be.
To start with, one has to chant more and more without faltering. That will verily take care of the rest.
--- H.H.SRI SRI MURALIDHARA SWAMIJI