Bandaranayake states that the religion seems to have achieved "undisputed authority" during the reigns of Dutthagamani and Vattagamani (c. mid-2nd century BCE to mid-1st century BCE). K. M. de Silva states that by the first century BCE, Buddhism was "well established in the main areas of settlement." De Silva also notes that as Buddhism was adopted by the Sinhalese, it assimilated pre-Buddhist cults, rituals and ceremonies. Buddhism became a powerful factor in the unification of Sri Lankan under a single political power with a unified culture.