Apostasy, or "falling away" is a danger that has been long in the making but has come front and center over the past few decades and seems prevalent today.
This is one of many such warnings in the New Testament about being ready for Jesus' return. The days are getting confusing - "clergy" telling believers that the rapture is a Biblical myth, that (despite the Bible clearly saying otherwise), that Christians will go through the Great Tribulation and days of judgement just as the rest of the world.