Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son.
(2 John 1:9 NKJV)

do not go beyond what is written

This passage, though not popular, is quite plain.

1. To transgress is ‘to step over...or across...to go beyond (a limit or boundary); to break a law, commandment, etc.; sin’(Webster’s New World Dictionary).  Other versions render it, Anyone who goes too far… NASB; Everyone who goes on ahead… ESV, etc.
2. One who goes beyond the Lord’s teaching (doctrine) necessarily severs his relationship with God. 
3. Contrary to popular opinion, true doctrine is very important, as this verse clearly shows.  Not only must we embrace truth, but also reject those who promote doctrines of men.   “If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him; for he who greets him shares in his evil deeds” (2 John 10-11).
4. This passage stands with many others that emphasize the same truth.
   1. The Lord commissions His apostles to meet this obligation to their converts:  “teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age” – Matthew 28:20.
   2. Paul cautions his readers in Colosse, “And whatever you do in word or deed, do all in the name of the Lord Jesus, giving thanks to God the Father through Him” – Colossians 3:17.
   3. Paul reminds Timothy, “As I urged you when I went into Macedonia—remain in Ephesus that you may charge some that they teach no other doctrine” – 1 Timothy 1:3.
   4. Paul further warns, “If anyone teaches otherwise and does not consent to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which accords with godliness” – 1 Timothy 6:3.
   5. The New Testament ends on this solemn note:  “For I testify to everyone who hears the words of the prophecy of this book: If anyone adds to these things, God will add to him the plagues that are written in this book; ¹⁹ and if anyone takes away from the words of the book of this prophecy, God shall take away his part from the Book of Life, from the holy city, and from the things which are written in this book” – Revelation 22:18-19.

Only those who abide in the doctrine of Christ can rightfully claim to have a saving relationship with God. 

-- Rick Duggin