1550s, from Satan + -ist. Ppplied by their enemies to Protestant sects. Meaning "Satan-worshipper" is from 1896.
1560s, "satanic disposition," from Satan + -ism. Applied 1820s-30s to the poetry of Byron, etc. Meaning "worship of Satan" dates from 1895, with reference to France, where it was said to be active at that time.