The third topic that John discusses is Doctrine.  II John 1: 7, 9-10a  "For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh.  This is a deceiver and an antichrist. … Whosoever transgresses and abides not in the doctrine of Christ, has not God.   He that abides in the doctrine of Christ, he has both the Father and the Son.  If there come any to you, and bring not this doctrine, receive him not…"

These people had two sources of knowledge.  Written letters, and "preachers" who came through their area. The most reliable knowledge was from letters written by the Apostles, which would include Paul. (Remember what I wrote you about Paul's apostleship.)  But there were others who came through saying that they had a "new word" concerning the gospel.  This was very confusing to these people.  Almost all of them were illiterate.  They couldn't read, and they couldn't write.  They were dependent on others to tell them what the truth was.  And the first theme that John emphasized was about truth.

So he reminded the lady and her children to beware of deceivers who changed what John had taught them.  He used very strong words.  Deceiver and Antichrist.

There are always people who want to add something or take something away from the simple gospel.  Which is that God became man.  He lived a perfect life, died as a payment for our sin, rose on the third day and is forever with the Father to intercede as our priest.  Not only are there prople who would change that simple gospel and add to it.  Others would invent a new religion altogether.  Doctrine is important.  You can believe in the wrong thing and end up lost.

You don't need a prayer cloth, a mustard seed, holy water or anything else to be close to God.

Just Jesus.