‘Apostasy’ is the term used to describe a revolt against and a departure from the truth of God. The use of the word in Acts 21:21 shows that it properly describes a forsaking of the great truths of the written Word. The written Word of God and nothing else constitutes the faith once delivered to the saints. We are warned in Scripture that there will come a falling away from the truth of God as the return of Jesus Christ approaches. The love of many shall wax cold!
The question to be addressed is: what is to be the Christian’s attitude to this falling away? We have the answer in the written Word of God.
1. We are to mark out those who depart from the faith. Romans 16:17 Now I beseech you, brethren, mark them which cause divisions and offences contrary to the doctrine which ye have learned; and avoid them. The Christian is to measure up all that they hear taught against the infallible standard of the Bible.
2. We are to reprove them. Ephesians 5:11 And have no fellowship with the unfruitful works of darkness, but rather reprove them. The child of God is not to remain silent and complicit with error, they are to speak out against the error, reproving those who teach it.
3. We are to reject them. Titus 3:10 A man that is an heretick after the first and second admonition reject. If there is no turning from error after repeated counsel then we must reject those who teach the error.
4. We are to have no fellowship with them. 2 Corinthians 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? We cannot remain in fellowship with any who teach error.
5. We are to separate from them. 2 Corinthians 6:17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you. Not being able to fellowship with them we must part company from them and join a Church that does hold to the faith once delivered to the saints.
Update - The Bible also teaches Secondary Separation, that is, it requires the Bible believer to separate from other believers who are walking disorderly: Now we command you, brethren, in the name of our Lord Jesus Christ, that ye withdraw yourselves from every brother that walketh disorderly, and not after the tradition which he received of us.…And if any man obey not our word by this epistle, note that man, and have no company with him, that he may be ashamed. Yet count him not as an enemy, but admonish him as a brother, 2 Thess 3:6,14,15.