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Apostasy is a turning away from one's beliefs. For followers of Christ it means turning away from the truth taught in the Bible. Paul prophesied the falling away of the church to the "fables" of paganism and empty philosophy. This is apostasy:
2 Timothy 4:3,4 For the time will come when they will not endure sound doctrine, but according to their own desires, because they have itching ears, they will heap up for themselves teachers; and they will turn their ears away from the truth, and be turned aside to fables.
I hope you have found this helpful.