(3 John 1:1) The elder unto the wellbeloved Gaius, whom I love in the truth.
John writes to Gaius who presumably is a bishop having oversight over a relatively small congregation, perhaps a couple of hundred. John refers to himself as the elder; perhaps there was also a younger John and he wanted to clearly identify himself.
John loves Gaius with the same love that Christians are to show toward one another.
(3 John 1:2) Beloved, I wish above all things that thou mayest prosper and be in health, even as thy soul prospereth.
John distinguishes between the body of Gaius and his soul. John assumes that his soul prospers, presumably because he is a Christian and the soul is the part of the person that gets saved. But John also wishes that Gaius may prosper in the material realm and be in good health — these are aspects of the body. Thus, John clearly distinguishes between the two aspects of a person: body and soul. Note that the soul is saved and that there is no distinction between soul and spirit: we are not three parts (body, soul, spirit) as some claim.
(3 John 1:3) For I rejoiced greatly, when the brethren came and testified of the truth that is in thee, even as thou walkest in the truth.
Gaius walks in the truth. The brethren spoken of are probably bishops and elders who had passed through wherever it is that Gaius ministered.
(3 John 1:4) I have no greater joy than to hear that my children walk in truth.
Gaius is a spiritual child of John. Therefore, it is proper to say that John is the spiritual father of Gaius. It is ok to call people father.
(3 John 1:5) Beloved, thou doest faithfully whatsoever thou doest to the brethren, and to strangers;
Gaius treats his own congregation and strangers with dignity and charity. He is a good pastor.
(3 John 1:6) Which have borne witness of thy charity before the church: whom if thou bring forward on their journey after a godly sort, thou shalt do well:
(3 John 1:7) Because that for his name's sake they went forth, taking nothing of the Gentiles.
(3 John 1:8) We therefore ought to receive such, that we might be fellowhelpers to the truth.
(3 John 1:9) I wrote unto the church: but Diotrephes, who loveth to have the preeminence among them, receiveth us not.
Diotrephes will not help out Christian leaders who pass through doing various tasks and ministering to Christians on their journeys.
John refers to himself and others working with him in guiding the Church as "us".
(3 John 1:10) Wherefore, if I come, I will remember his deeds which he doeth, prating against us with malicious words: and not content therewith, neither doth he himself receive the brethren, and forbiddeth them that would, and casteth them out of the church.
John refers to this congregation led by the schismatic Diotrephes as "the church". If it is a church it is not because its leader Diotrephes was ordained via the sacrament of Holy Orders but, rather, because there were still in the congregation God-loving Christians having faith in Jesus.
Notice John's use of the word "church". He seems to be using it to mean merely "assembly" rather than a divinely-ordained institution.
Diotrephes is speaking badly of those having apostolic approval to minister to the Church. Thus we see that the key ingredient of recognizing truth in the early church was apostolic approval. Those who rejected the apostles were always schismatics or worse. The church Diotrephes led was not a true church. Hopefully, John will inform the Christians at large what to do when their leader is corrupt or unholy; certainly the present-day Catholic Church proves to be unhelpful in this regard.
The Catholic Church teaches the only thing needed to have a true Church is a priest and bishop ordained via the sacrament of Holy Orders; that it doesn't matter whether they are schismatic or even heretics. This definition of a Church is madness!
(3 John 1:11) Beloved, follow not that which is evil, but that which is good. He that doeth good is of God: but he that doeth evil hath not seen God.
We are not to follow bad spiritual leaders because they are not of God. Perhaps Christians in churches led by such as Diotrephes should find somewhere else to go. In any case they should stop attending churches having bad leaders who reject apostolic authority and who teach error.
It almost sounds like John is trying to persuade Gaius not to become like Diotrephes. This kind of verbiage from apostles is common, for example Paul exhorting Timothy to remain true to the faith.
(3 John 1:12) Demetrius hath good report of all men, and of the truth itself: yea, and we also bear record; and ye know that our record is true.
John gives a good report of Demetrius and testifies that he teaches the true gospel; you could say that his teachings are infallible. John also reminds them that his own teaching is trustworthy and true, but note that he refers to it as "our record". He seems to be including all the bishops he has charge over and testifying that their teaching is trustworthy and true also. Apparently, Demetrius was in some other organizational category from all these so he singled him out. Perhaps Demetrius was a prophet who wandered in and out of their territory or perhaps he was merely the one who delivered the letter (but it is hard to explain why this alone would have gained him such high praise about the trustworthiness of his teaching, even above those with John).
(3 John 1:13) I had many things to write, but I will not with ink and pen write unto thee:
Not everything important is in the Bible. Apostolic teaching is also contained in the writings of the early church fathers, especially those personally discipled by an Apostle. But after 20 centuries we can only trust things written down long before. This is a flaw in Catholic teaching; they claim oral traditions are sources of truth.
(3 John 1:14) But I trust I shall shortly see thee, and we shall speak face to face. Peace be to thee. Our friends salute thee. Greet the friends by name.
King James Version