1, 2 and 3 John
All of God's word deals with God's eternal purpose and plan; we need to break down God's word in line with God's plan and purpose. 1, 2, and 3 John all have truths that are based on the books that are placed before it: Hebrews, James, 1 and 2 Peter. It is the "milk to meat" edification concept.
All the Books in this series, Hebrews through Revelation, all express the idea of the believer being able to tell the difference between the remnant and the spirit of the Antichrist. The purpose of John's Epistles are to prevent the true believer from the seduction of the false teachers and prophets.
John writes to the believers that what he says is true even though there will be others that are saying things contrary, the most important is that of Christ's manifestation in the flesh. Even Peter writes that the nation of Israel will be told that what the remnant is believing is false.
There is much emphasis in the Epistles of John for the believer to be aware of who is of God and who is not. They will be able to tell by what is spoken (see James 3).
In the time of the Tribulation, there will be pressure to turn back to the old way of worship and the remnant is warned not to do so for that old system is now under the spirit of the Antichrist. God calls that religious system vain for it:
God has a new way of doing things, the old has passed away. John warns about the sin unto death, which is to turn back into the old system which is now led by the Antichrist. The remnant believers will not be doing what the rest of the nation of Israel is doing.