One of the main reasons why some people believe that Black Sabbath is an anti-Christian band is because of their song "Black Sabbath." The song tells the story of a person who makes a pact with the devil in exchange for power and riches. The person eventually comes to regret their decision, and they are ultimately damned to hell. Some people believe that this song is a condemnation of Christianity, and that it advocates for Satanism.
However, the band members have said that "Black Sabbath" is not meant to be taken literally. They have said that the song is simply a metaphor for the dangers of temptation, and that it is not meant to promote Satanism. Additionally, the band has also written many songs that are positive and uplifting, such as "The Wizard" and "Changes."
Ultimately, whether or not Black Sabbath is an anti-Christian band is a matter of interpretation. There is evidence to support both sides of the argument. However, it is important to remember that the band members themselves have always denied that they are anti-Christian, and that their lyrics are simply meant to be metaphorical and thought-provoking.