Hebrews 4:11-13

11 Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience. 12 For the word of God is living and powerful, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart. 13 And there is no creature hidden from His sight, but all things are naked and open to the eyes of Him to whom we must give account.

The Word of God is a tool of discernment to determine the thoughts and intents of the heart. Scripture cuts both ways, and divides the spiritual from the non-spiritual.

Hebrews 5:11-14

11 of whom [Melchizedek] we have much to say, and hard to explain, since you have become dull of hearing. 12 For though by this time you ought to be teachers, you need someone to teach you again the first principles of the oracles of God; and you have come to need milk and not solid food. 13 For everyone who partakes only of milk is unskilled in the word of righteousness, for he is a babe. 14 But solid food belongs to those who are of full age, that is, those who by reason of use have their senses exercised to discern both good and evil.

Spiritual maturity comes through becoming skilled in the Word of Righteousness by way of exercising and disciplining the human faculties of perception to discern and divide between good and evil – the thoughts and intents of the heart.

2 Timothy 2:14-18

14 Remind them of these things, charging them before the Lord not to strive about words to no profit, to the ruin of the hearers. 15 Be diligent to present yourself approved to God, a worker who does not need to be ashamed, rightly dividing [literally: cutting straight, that is, handling accurately] the word of truth. 16 But shun profane and idle babblings, for they will increase to more ungodliness. 17 And their message will spread like cancer. Hymenaeus and Philetus are of this sort, 18 who have strayed concerning the truth, saying that the resurrection is already past; and they overthrow the faith of some.

The worker who cuts straight – handles accurately – the Word of Truth has nothing to be ashamed of, being approved by God. Scripture must be cut with Scripture – we cannot trust ourselves to accurately handle and interpret any part of Scripture apart from the remainder of Scripture.

2 Timothy 3:14-17

14 But you must continue in the things which you have learned and been assured of, knowing from whom you have learned them, 15 and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus. 16 All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, 17 that the man of God may be complete [literally: fully functional], thoroughly equipped for every good work.

Scripture is God-breathed – it is living and active because it has the life of God breathed into it. Scripture is both necessary and sufficient to make a man of God fully functional and thoroughly equipped for every good work.

2 Peter 1:12-21

12 For this reason I will not be negligent to remind you always of these things, though you know and are established in the present truth. 13 Yes, I think it is right, as long as I am in this tent, to stir you up by reminding you, 14 knowing that shortly I must put off my tent, just as our Lord Jesus Christ showed me. 15 Moreover I will be careful to ensure that you always have a reminder of these things after my decease. 16 For we did not follow cunningly devised fables when we made known to you the power and coming of our Lord Jesus Christ, but were eyewitnesses of His majesty. 17 For He received from God the Father honor and glory when such a voice came to Him from the Excellent Glory: “This is My beloved Son, in whom I am well pleased.” 18 And we heard this voice which came from heaven when we were with Him on the holy mountain. 19 And so we have the prophetic word confirmed [Literally: we {apostles} have a more firm / sure / certain prophetic word], which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; 20 knowing this first, that no prophecy of Scripture is of any private interpretation, 21 for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit.

The Scriptures of the apostles are more firm, sure, certain than direct eye-witness visions and audible testimony from God. Scripture was not conveyed by man’s will through private interpretations of visions or the like, but the Scriptures were conveyed by the Holy Spirit through holy men.

2 Peter 3:1-2, 15-16

1 Beloved, I now write to you this second epistle (in both of which I stir up your pure minds by way of reminder), 2 that you may be mindful of the words which were spoken before by the holy prophets, and of the commandment of us, the apostles of the Lord and Savior, … 15 … as also our beloved brother Paul, according to the wisdom given to him, has written to you, 16 as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught [literally: un-discipled] and unstable [/weak and unsteady] people twist [/literally: torture to make a false confession] to their own destruction, as they do also the rest of the Scriptures.

The apostles were self-aware that their epistles were Scripture, eventually to be distributed among all Christians. The Scriptures – particularly more difficult ones – can be tortured to say things which they do not mean, hence we must strive to be stable and steady in the Word.