All Scripture is given by the inspiration of God and Holy Spirit moved the authors to write down what God wanted to give as written Word to mankind.
So Paul’s teachings are as credible as Jesus teachings and Peter’s. There is no separation in the Word. In other words, Bible is from the mouth of God and God used the prophets and apostles and authors of the books as scribes. God also orchestrated the historical events that went into the Bible.
2 Tim 3:16 “All scripture [notice the singular, not scripture’s’] is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:”
2 Pet 1:21 “For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.”
2 Pet 3:15-16 “And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you; 16As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.”
‘Unlearned and Unstable’ are those who do not have Spirit of Christ, the Carnal [unsaved individuals though very learned in theology], who interpret the scriptures with human wisdom. The only Biblical methodology for ‘sound doctrine’ is to interpret scriptures by ‘comparing spiritual with spiritual’/comparing scripture with scripture. This is how the Holy Spirit guides a Spiritual [saved individual] into Truth.
1 Cor 2:13 “Which things also we speak, not in the words which man’s wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.”
1 Cor 3:1-2 “And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ. 2I have fed you with milk, and not with meat: for hitherto ye were not able to bear it, neither yet now are ye able.”
2 Cor 1:12 “For our rejoicing is this, the testimony of our conscience, that in simplicity and godly sincerity, not with fleshly wisdom, but by the grace of God, we have had our conversation in the world, and more abundantly to you-ward.”
What corrupts ‘the simplicity that is in Christ’? The doctrines of men, the traditions and the fables, anything that is not in accordance with the Bible, corrupt the Gospel. Any doctrine on any subject of the Bible should be in full harmony with the whole teaching of the Bible on that subject.
2 Cor 11:2-4 “For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ. 3But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ. 4For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.”
Mark 7:5-9 “Then the Pharisees and scribes asked him, Why walk not thy disciples according to the tradition of the elders, but eat bread with unwashen hands? 6He answered and said unto them, Well hath Esaias prophesied of you hypocrites, as it is written, This people honoureth me with their lips, but their heart is far from me. 7Howbeit in vain do they worship me, teaching for doctrines the commandments of men. 8For laying aside the commandment of God, ye hold the tradition of men, as the washing of pots and cups: and many other such like things ye do. 9And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.”
Gal 4:9-11 “But now, after that ye have known God, or rather are known of God, how turn ye again to the weak and beggarly elements, whereunto ye desire again to be in bondage? 10Ye observe days, and months, and times, and years. 11I am afraid of you, lest I have bestowed upon you labour in vain.”
1 Tim 1:4 “Neither give heed to fables and endless genealogies, which minister questions, rather than godly edifying which is in faith: so do.”
2 Tim 4:3-4 “For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears; 4And they shall turn away their ears from the truth, and shall be turned unto fables.”
God’s Word is Truth and has more validity than eyewitnesses.
2 Pet 1:16-21 “For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. 17For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. 18And this voice which came from heaven we heard, when we were with him in the holy mount. 19We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts: 20Knowing this first, that no prophecy of the scripture is of any private interpretation. 21For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.”
God expects a believer to Grow in Meat of the Word and not stuck on Milk of the Word.
Pro 1:22-23 “How long, ye simple ones, will ye love simplicity? and the scorners delight in their scorning, and fools hate knowledge? 23Turn you at my reproof: behold, I will pour out my spirit unto you, I will make known my words unto you.”
1 Pet 2:2 “As newborn babes, desire the sincere milk of the word, that ye may grow thereby:”
Heb 5:12-14 “For when for the time ye ought to be teachers, ye have need that one teach you again which be the first principles of the oracles of God; and are become such as have need of milk, and not of strong meat. 13For every one that useth milk is unskilful in the word of righteousness: for he is a babe. 14But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.”
God’s Word is the Sword in the hand of the believer that protects from Satan’s deception especially in these last days as Satan comes as an angel of light and his ministers as ministers of righteousness.
John 8:30-32 “As he spake these words, many believed on him. 31Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed; 32And ye shall know the truth, and the truth shall make you free.”
2 Cor 11:14-15 “And no marvel; for Satan himself is transformed into an angel of light. 15Therefore it is no great thing if his ministers also be transformed as the ministers of righteousness; whose end shall be according to their works.”
Eph 6:10-17 “Finally, my brethren, be strong in the Lord, and in the power of his might. 11Put on the whole armour of God, that ye may be able to stand against the wiles of the devil. 12For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. 13Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand. 14Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness; 15And your feet shod with the preparation of the gospel of peace; 16Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked. 17And take the helmet of salvation, and the sword of the Spirit, which is the word of God:”