How do those who cite 2 Tim 3:16 to prove the inspiration of Scripture know that 2 Tim 3:16 is itself inspired and what is meant by "All Scripture"?

I write from an evangelical perspective.

The question has several ideas wrapped together. This answer attempts to show what, in general, are the expected features of a divinely inspired work, and that 2 Timothy 3:16 cannot be used, by itself, to prove the divine origin of scripture. Such verses are necessary but not sufficient.

There are probably many books which are useful in religion: The Imitation of Christ by Thomas a Kempis, the Hindu Upanishads and many other Hindu works, the writings of Confucius and Buddha. But as far as I remember none of these works specifically claim to be "given by inspiration of God". (You could argue that The Imitation is dressed in the language of God talking to a believer, so I guess it is debatable.) These works do not need scrutiny to see if they are from God: they never claim it, we can thus read them and judge them upon their own merits.

But there are other works, such as the Bible and the Qu'ran which do claim to be given by God. Such works invite much closer scrutiny.

Obviously, if God were to inspire a work especially a work telling us the way of salvation he would tell us it was from him. This is because God is good and wise. Any claim in any work is not decisive. The claim is necessary, but not sufficient.

By what criteria do we measure such a work? In this reply I shall only look at the Bible.

(1. Does it claim to be inspired by God? (If it does not then it cannot be from God.))

2. Is it internally consistent in its teaching? The Bible was written over 1500 years by many different authors yet has one consistent message: we are all sinners, under the condemnation of a Holy God, and we need God to become a man and rescue us by his death as an atoning sacrifice.

3. Is it externally consistent in its teaching? Does its teaching square up with the world around us? The world is full of evil: this is what the Bible teaches.

4. What is the standard of its moral code? The Bible teaches the highest possible moral code: that we should love God, our neighbour and our enemies. It teaches we should never bear false witness, we should speak the truth, and that "all liars shall have their part in the lake which burns with fire and brimstone which is the second death" (Rev 21:8). How can a book which says such things as these on the one hand be deceiving to us about its divine origin on the other hand?

5. If it delves into other fields, such as history or science, is it accurate in those fields?

6. Are most people keen to read it? If most people are sinners then if they are OK with reading this work then it cannot be from a Holy God.

7. Does God vouch for it? What effect does it have on those who do read it and follow it truly? A tree is known by its fruit, so does it produce good fruit in those who follow it rightly?

8. Does the Devil vouch for it? Does he produce many false religions which claim the Bible for their final authority?

9. Does the work promote other-worldliness or heavenly-mindedness? If our chief need is to truly get right with God, then is this the chief focus of the book or does it focus on building an earthly religious organisation or building a worldly power on earth for its followers? God surely looks at our hearts: does the work focus on our inner life, inner thoughts affections and behaviours, or does it focus on externals of behaviour. Surely any work from God must promote and value the inner life.

10. Finally, though I expect I have missed many key requirements, does the work actually work? Does its teaching change the inner man? That the God of Heaven gave the best of Heaven, his only begotten Son, to suffer unimaginable internal anguish, taking our sins fully upon himself, that we might have all our sins forgiven even when we were at our very worst, this melts our hearts and leads us to repentance and faith in the Saviour of the world. It fills us with gratitude, and makes us realize there is nothing in the whole world compared to this. Now I want to live for Him who loved me and gave himself for me.

We receive the letter of 2 Timothy as inspired because the first century church received it as from the Apostle Paul (and thus as inspired), and we know that because no first century objections were raised as to its authenticity, and the letter was faithfully handed down to the next generation of the church as authentic.

This lack of any extant opposition to the letter from the first century (and lack of any first century oppostion to all of Paul's writings) is all the more surprising in view of the existence of enemies of Paul (and his doctrine), the "Super Apostles", within the church.

Neither is it necessary for the first century church to positively vouch in favour of the writings we attribute to Paul. Their silence is enough.

The question suffers from a false assumption : that salvation is through scripture, not the Lord himself.

Jesus said, Ye search the scriptures for ye think that in them ye have eternal life, John 5:39. Eternal life is not obtainable by mere study of the written record which God has provided.

'Salvation is of the Lord', Psalm 3:8, Psalm 62:1, Jonah 2:9.

It is the Lord himself who sends a man whose name is John. That is to say John the Baptist. And this man cited as 'John' does not just appear in that generation. The ministry of the baptism of repentance is sent in every generation by the Lord himself.

It is that ministry that speaks 'thus saith the Lord' and it is that ministry that speaks cogent and pertinent truth to the soul which brings the soul into convictions and works an awakening in the soul to spiritualities.

It doesn't happen just 'with the bible'.

There was a man sent from God.

This is the salvation of the Lord.

And it is perfectly clear throughout the Old and New testaments that God repeatedly sends his prophets and apostles and ministers (who follow in the steps of the apostles) to minister to the people.

Thus does the Shepherd call his sheep. And the sheep hear his voice and they follow him. They do not hear hirelings and thieves and wolves in sheep's clothing. They hear the voice of the shepherd.

Because they are sheep. And not goats.

This has been true down through the centuries since Jesus Christ arose from the dead. The Lord sends his servants. And they preach the gospel. And the sheep hear the voice of the Lord calling them within that preaching.

And they follow the Lord.

And they are turned to a book that is unique above all books. Because the voice of the Lord can be heard through that book. Some pretend to have written books, that are not from the Lord, mimicking the Lord's book. But the sheep can tell.

They hear the voice of the true shepherd.

The doctrine of Christ is like meat to their soul. They feed among the lilies, Song 4:5. They have no appetite for that which is not good food. Inadvertently they may swallow down some foul matter, but they will vomit it up again.

These are the footsteps of the flock.

They follow men who were said to be ignorant and unlearned men. But these men had been with Jesus, Acts 4:13. And Jesus had chosen them and separated them and called them to a ministry and a service, Matthew 4:19.

This is life, not just words on a page.

Even when they go astray they hear a voice :

And thine ears shall hear a word behind thee, saying, This is the way, walk ye in it, when ye turn to the right hand, and when ye turn to the left. [Isaiah 30:21 KJV]

And One is sent to them to guide them :

Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come. [John 16:13 KJV]

God himself is their teacher and he watches over them :

I will instruct thee and teach thee in the way which thou shalt go: I will guide thee with mine eye. [Psalm 32:8 KJV]

He watches over them constantly :

Behold, he that keepeth Israel shall neither slumber nor sleep. [Psalm 121:4 KJV]

The sheep will be guided by all means and every means and they will be guided into all truth, they will be guided as to what to read and how to read it and what to avoid reading.

Many shall stumble and fall and turn aside to false ways, but not the sheep of Christ :

A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee. [Psalm 91:7 KJV]

Postscript : The Methods of Socrates

Plato described this rigorous method of teaching to explain that the teacher assumes an ignorant mindset.

Scholars such as Peter Boghossian suggest that although the method improves creative and critical thinking, there is a flip side to the method. He states that the teachers who use this method wait for the students to make mistakes. Socratic Method

It is interesting that someone should regard themselves as a teacher and then implement Socratic methods in order to 'teach'.

Let every man be swift to hear and slow to speak, is the way of Christ, James 1:19.