”Do not go beyond what is written” is an excellent admonition for us today as well. God is omnipotent and doesn’t need anyone to add anything to His written word. In fact, He warns us against doing just that.
18 I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book;–Rev. 22:18 NASB
God also warns us against adding any other requirement for receiving salvation.
10 Now therefore why do you put God to the test by placing upon the neck of the disciples a yoke which neither our fathers nor we have been able to bear? 11 But we believe that we are saved through the grace of the Lord Jesus, in the same way as they also are.”—Acts 15:10, 11 NASB
These instructions are so all of us will be preaching and teaching the same message. The message is that when Jesus died for our sins and said “it is finished”, there wasn’t and still isn’t anything else needed. We are all the same in that we were all born in sin and need a Savior. We received salvation by grace through faith, not by anything we did or could do. Since all we did was receive a gift, what reason could we possibly have for boasting?