The word apostasy comes from the Greek apostasia (Strong's Concordance #G646) which, in the New Testament, means a falling away or defection from, or forsaking of, God's truth. The English word is not found in the King James Bible. The Greek, however, is found in only two places. In Acts 21:21, apostasia is translated as "to forsake" and in 2Thessalonians 2:3 as "a falling away."
Apostasy is not "backsliding" or the setbacks all Christians experience from time to time as they try to live God's way. It is also not the denial of God or Jesus in a moment of weakness, like Peter's denial when the Lord was arrested (Luke 22:31 - 34, 54 - 62). Apostasy, rather, is the willful rejection of faith in Christ by those who previously had professed it (see 1Timothy 1:19 - 20, Hebrews 3:12 - 14, 2Peter 2:20 - 21). Other religions, like Judaism, consider it occurring when someone renounces their faith.
An old example
Apostasies in the Old Testament took place when Israel rejected the laws and precepts given to her by God (the Old Covenant). For example, one notable rejection took place when King Ahab and Jezebel zealously promoted the worship of pagan deities. Their behavior, which influenced countless others into forsaking the Lord, brought three years of punishment on Israel (1Kings 16:29 - 17:1).
New Testament examples
Paul's ship, during the last part of his third missionary journey, anchors in Miletus. After requesting Ephesian church elders visit him, he warns them about a coming apostasy in the church. He even goes so far as to reveal that some of them will lead people away from the truth for their own selfish desires (Acts 20:26 - 31).
When Paul, in 58 A.D., arrived in Jerusalem, he was met by James and other church elders. They revealed that many Jews believed Paul's teachings were promoting apostasy from God's laws and customs (Acts 21:21). The church's "fix" to correct such misconceptions, however, ended up getting Paul arrested!
Jewish converts to Christianity, in the book of Hebrews, are alerted about the danger of apostasy. They are not only chided for their slow spiritual growth but also told of the possibility of losing everything they had gained (Hebrews 5:11 - 6:20).
End Time rebellion
Seeking to tap down Thessalonica's growing expectation of Jesus' imminent return, Paul reveals that an spiritual rebellion must occur before the Second Coming.
Do not let anyone deceive you by any means because that day will not come unless the apostasy shall come first, and the man of sin shall be revealed - the son of perdition . . . (2Thessalonians 2:3).
Did you know . . .
The Catholic Church designates three major kinds of rejection of their faith. According to the 1913 Catholic Encyclopedia they are the following.
Apostasy a Fide, which occurs when a Christian (i.e. Catholic) completely and voluntarily abandons their faith.
Apostasy ab ordine, which happens when a cleric, who has received major orders, abandons the clerical dress and ecclesiastical state.
Apostasy a religione, which takes place when a religious person leaves the religious life (e.g. monastery) with the intention of never returning to it.