The sutras say that “there can be no fault so serious that it cannot be purified by the four powers or four opponent forces.” Even Milarepa and Angulimala, who were both mass-murderers, were able to purify their negative karma and become enlightened in a single lifetime, but they both were able to do this by following enlightened masters and using these powers. These four opponent powers that are essential for successful purification are:
1) The power of regret: One begins by reflecting on regret; the awareness that actions we have committed bring suffering to ourselves and others. This is not the same as guilt, which implies a negative and helpless state of mind and is not useful, but a sort of “intelligent regret,” which is a very positive and creative mental state aimed at correcting the mistake so we won’t repeat it.
2) The power of reliance: To correct our mistakes or negative actions directed toward either the Three Jewels or other sentient beings, we take refuge and generate bodhichitta. We rely on the Buddha who is our role model, the dharma that is the teachings of the Buddha, and the sangha.
3) The power of remedy (the antidote): These are positive actions of body, speech, and mind that we do to purify the negativity. This is “building the wall” of good karma. This can include kind deeds, chanting mantras, meditation, etc. and the dedication of the merit of what we have done to help anyone we may have harmed.
4) The power of resolve: This is our ongoing determination to never repeat the negative action and then not doing it again.
Of course there are dharmas that can be learned to help elicit the support of the Buddhas and Bodhisattvas, but they all depend on these four powers.