The Prophet Muhammad (S): Verily the inmates of the Fire will be tormented by the stench of the knowledgeable man who has abandoned his knowledge. The person with the most intense remorse on the Day of Resurrection will be the man who called someone to Allah and he responded to his call, accepted from him, became obedient to Allah and thus Allah admitted him into His paradise, while the caller was made to enter the Fire for abandoning his knowledge and following his whims and desires.