From the twelfth to the sixteenth century, when Buddhism, itself deeply infected with Tantrism, was disappearing, Śâktism was probably the most powerful religion in Bengal, but Vishnuism was gaining strength and after the time of Caitanya proved a formidable rival to it. At the beginning of the fifteenth century we hear that the king of the Ahoms summoned Brahmans to his Court and adopted many Hindu rites and beliefs, and from this time onward Śâktism was patronized by most of the Assamese Rajas although after 1550 Vishnuism became the religion of the mass of the people. Śâktism never inspired any popular or missionary movement, but it was powerful among the aristocracy and instigated persecutions against the Vishnuites. The more respectable Tantras[699] show considerable resemblance to the later Upanishads such as the Nṛisinhatâpanîya and Râmatâ-panîya, which mention Śakti in the sense of creative energy.[700] Both classes of works treat of magical formulæ (mantras) and the construction of mystic diagrams or yantras. This resemblance does not give us much assistance in chronology, for the dates of the later Upanishads are very uncertain, but it shows how the Tantras are connected with other branches of Hindu thought. The distinction between Tantras and Purâṇas is not always well-marked. The Bhâgavata Purâṇa countenances tantric rites[701] and the Agni Purâṇa (from chapter XXI onwards) bears a strong resemblance to a Tantra. As a rule the Tantras contain less historical and legendary matter than the Purâṇas and more directions as to ritual. But whereas the Purâṇas approve of both Vedic rites and others, the Tantras insist that ceremonies other than those which they prescribe are now useless. They maintain that each age of the world has its own special revelation and that in this age the Tantra-śâstra is the only scripture. Thus in the Mahânirvâṇa Tantra Śiva says:[702] "The fool who would follow other doctrines heedless of mine is as great a sinner as a parricide or the murderer of a Brahman or of a woman.... The Vedic rites and mantras which were efficacious in the first age have ceased to have power in this. They are now as powerless as snakes whose fangs have been drawn and are like dead things." The Kulârṇava Tantra (I. 79 ff.) inveighs against those who think they will obtain salvation by Vedic sacrifices or asceticism or reading sacred books, whereas it can be won only by tantric rites.