An antichrist is an opponent of Christ; he is one who is in opposition to the true gospel, the true Church, and the true plan of salvation. (1 John 2:19; 4:4–6.) He is one who offers salvation to men on some other terms than those laid down by Christ.
Sherem (Jac. 7:1–23), Nehor (Alma 1:2–16), and Korihor (Alma 30:6–60) were antichrists who spread their delusions among the Nephites.
(Bruce R. McConkie, Mormon Doctrine, p. 39)