"Reciting OM MANI PADME HUM purifies negative karma – all the obscurations – collected from beginningless rebirths. It is unbelievable. Even by reciting the mantra one time, a fully ordained monk who has broken the four root vows (to not have sexual intercourse, steal what belongs to others, kill a human being, or tell lies that he has realizations while he doesn’t have them) is purified. Just reciting OM MANI PADME HUM purifies the five heavy negative karmas (it saves from being born in the lower realms) of even having killed this life’s parents; caused blood to flow from a buddha; killed an arhat, who is free from samsara, who has removed the causes of samsara – delusion and karma; and caused disunity among the Sangha. "
Lama Zopa Rinpoche.