While it is true that Miller admits that there is some truth behind the accusations of those who worshipped the Devil in Salem (and elsewhere around the world), it should be made abundantly clear that these dabblings in occultism veil a larger, more insidious "threat": that of sensuality and personal liberty.
If you delve a bit farther into the speech Bmadnick rightly mentions, and consider the play as a whole, the daliances of the girls and Tituba are small potatoes compared to the larger threat of oppression, fear of the "other" and fear of sex in general.
Miller wrote this play in response to the McCarthy "witch hunts" which have uncomfortable parallels to the events of Salem. In those troublesome days of the 1950s, the "threat" was Communism, or the "Reds" (in Salem, the "Devil'). At the heart of both was ignorance, as well as losening standards for women's "acceptable" behavior (post WWII "back to the kitchen" mentality).
At the end of Miller's narrative (after Hale's entrance) he writes, "Our opposites are always robed in sexual sin, and it is from this unconscious conviction that demonalogly gains both its attractive sensuality and its capacity to infuriate and frighten."
We fear the "other"; we fear change. It is these elements, not a few people who "worship the devil" that are the perceived "threat."