The answer is simple. When a person gives up his enmity, the desire for revenge and settling scores vanishes, the chain is broken. Hence one should be forgiving in life.
Saints tell us that sin is caused by lust and that ignorance is the cause of lust. As long as there is lust, the cycle of birth and death is unending. In order to get rid of lust we need to get rid of our ignorance. And the dusk of ignorance is conquered by the dawn of knowledge.
Knowledge manifests by the grace of a Sadguru, the true preceptor. When all our Karmas (deeds) are destroyed by the fire of true knowledge, the cycle of birth and death is destroyed and there is no more rebirths.
Just as a burnt seed does not sprout even when sowed in fertile soil, the votary whose desires and Karmas have been burnt in the flame of knowledge is liberated from the cycle of birth and death.
