“Heresy” is false doctrine. It is a belief or teaching that is not biblical. An example would be saying that people save themselves by the things they do in life. The Bible teaches that people enjoy God’s free gift of salvation through faith in Jesus Christ alone (Ephesians 2:8-9; Titus 3:5-6).
Because heresies are so serious, the prayer of the hymn writer is very much in place: “O gracious Lord, direct us, Your doctrine pure defend, From heresies protect us, And for your Word contend, That we may praise you ever, O God, with one accord And say: The Lord our Savior Be evermore adored!” (Christian Worship 585:3)