Not all Rastas smoke weed, that is a common misconception. Some frown upon it quoting biblical scripture - somethin bout "Dont make your temple in2 a chimney" or some shit. However, Rastas who do use the herb use other various biblical scripture as well to "aid" their reasoning.
And Rastafarianism is a sect of Orthodox Ethiopian Christianity formed out of Jamaica, largley [and unintentionally] by Marcus Garvey in the late 20s/early 30s. Was inspired by his "Back to Africa" movement, and an apparent fulfillment of his "prophecy" when Prince Ras Tafari was crowned Emperor of Ethiopia