Eradicate Karmic Obstructions and Avoid Calamities by Reciting Mantras
Some may question whether the idea of eradicating karmic obstructions and avoiding calamities by reciting mantras appears to contradict the doctrine of cause and effect in Buddhism.
Reduce Bad Deeds and Develop Good Karmic Roots
In fact, to uphold a mantra by reciting it with a pure and concentrated mind is to observe the precepts in and of itself. Doing so will reduce the chances for us to do bad deeds, transform our negative thoughts, and help us cultivate our good karmic roots. Provided that we continuously recite a mantra and take every chant as an invocation to the Buddhas or Bodhisattvas mentioned in the mantra, the cumulative and complementary effects of both the mantra and our concentrated mind must correspond with their fundamental vow power. The Buddhas and bodhisattvas, in turn, will safeguard us in our practice and help us go through difficult times in our life.
Lighter Retribution from Great Misdeeds
In addition, although karmic retribution resulting from great misdeeds is very hard to alter, sincere repentance makes it possible to receive a lighter retribution, due to the influence of new conditions on existing causes and effects. It is analogous to a time bomb: if someone knows how to defuse the bomb, a crisis can be averted. Therefore, fixed karma is not necessarily unchangeable.
50 Questions about Buddhist Dharanis and Mantras (咒語50問), Dharma Drum Publishing Corporation