Apostasy (apostasiā - Strong's Concordance no.646) is what occurs when a person, group or nation abandons their faith. This word appears several times in the Bible, but it always translated or 'rendered' as something else. We see it as a falling away in Paul's second letter to the church at Thessalonica, when he gives the follow advice:
2 Thessalonians 2:1-3 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, (2) That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand. (3) Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition.
Many have begun to fall away, some slowly, others quickly as they exchange God's truth and scriptural teaching of a Creator God with His Sabbath for evolution and spiritual Babylon, discarding His moral and social principles for secularism and humanism.
Apostasy is the opposite from what followed the preaching of Peter on the day of Pentecost, when "about three thousand souls" were added to the church, "and they continued steadfastly in the apostles' doctrine (teachings)..." without a hint of apostasy.
Peter then goes on to say some more about apostasy in his second general letter to the churches:
2 Peter 2:20-23 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. (21) For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them. (22) .
The Bible predicted that a great desolating power was to arise in later centuries that would seek to destroy the atonement and God's laws from among His people (Dan 7.8, 20-2 I, 25, 8:9-12).
The attempt by this power to change God's laws, and especially His Time which is embodied by a specific day set aside in His Law, was specifically predicted in Daniel 7:25. Only God can change the law, and so Paul predicted the rise of a man who would call himself God (2 Thess 2:3-4). With boldness this power would sit in the temple of God and call itself God (2 Thess 2.4), and boastfully admit what it had done, declaring it to be the Mark of its authority?
Those who love God acknowledge and honor God 's authority, and you galdly obey His commands and encourage others to do so. But when someone falls away or apostatises, they turn away or declare their independence of God when they set aside His law and refuse to keep it. They put themselves completely in the camp of Babylon as they set another as a rival god and a false spirit, when having set aside God's law. The Evil one comes and establish's in its place a counterfeit system of religion and then require others to keep it in place of the law that God commanded.
"Whom ye obey, his servants ye are." (Rom 6:16). God's Word declares that obedience to this man-made god by keeping his counterfeit day of worship, while knowing that there is not one word, not one hint in all of Scripture to keep that false day in place of the true Bible Sabbath--will soon bring upon oneself under allegience to the Evil one (Rev 13:16-17, 14:6-12). Only the Remnant who keep the commandments of God and the faith of Jesus will at that time resist it (Rev 13:8, 14:6-12, 12:17). The Bible predicts a return to the true Sabbath for God's people who stand fast till the end in the law of God--by again keeping His true Sabbath (Isa 58:13-14). And the prophecy is given that the saved of all ages will one day soon, honor the holy Sabbath of God for all eternity in the new earth (Isa 66:22-23).
Summary: When Jesus enters our heart, He shows us why we should love Him and let Him change our lives with his Holy Spirit so we love His Commandments and follow His Testimonies. Why should we want to turn back by apostising, why turn away from God's truth? What does Babylon really have to offer in comparison?