The word Rasta is an abbreviation of Rastafari, the name of both a religion based on the Old Testament and the life of Emperor Haile Selassie I of Ethiopia and adherents of the same.
This word has no categorical antonyms.
However one could loosely use the terms atheist or agnostic as antonyms for one who follows Rastafari, or the terms irreligion, secularity, or atheism as antonyms for the religion.