According to the teachings of Hinduism, whenever the forces of darkness would get the upper hand on earth, Vishnu would come to the aid of humanity by taking incarnation as an avatar. (In this sense of the word, Jesus Christ is an incarnation of Vishnu. So are Lord Maitreya, Lord Gautama Buddha, Lord Sanat Kumara. All come in the lineage of the Second Person of the Trinity, the Universal Christ.) The avatar would vanquish evil and establish the religion of the age in which he was born.