How do we know, from the Bible alone, that the letters of St. Paul, who wrote to first-century congregations and individuals, are meant to be read by us as Scripture 2000 years later?

12) How do we know, from the Bible alone, that the letters of St. Paul, who wrote to first-century congregations and individuals, are meant to be read by us as Scripture 2000 years later?

It was explained in Questions 8-11 that God authenticates His Word and that is not a violation of sola scriptura.

Apostolic doctrine tells us all scripture God inspired is profitable for instruction into the faith and were written for our learning:

2 Timothy 3:16 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 perfect, throughly furnished unto all good works.

Romans 15:4 For whatsoever things were written aforetime were written for our learning, that we through patience and comfort of the scriptures might have hope.

For example, the Ten Commandments written about 1,445 BC, was very much ad hoc, specific to the Israelites, yet Paul and the other writers of Scripture expected everyone pay it heed:

Exodus 20:2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
KJV Romans 3:29 Is he the God of the Jews only? is he not also of the Gentiles? Yes, of the Gentiles also:

Exodus 20:3 Thou shalt have no other gods before me. 4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: 5 Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; 6 And shewing mercy unto thousands of them that love me, and keep my commandments.
1 Corinthians 8:4 As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one.

Exodus 20:7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
James 5:12 But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.

Exodus 20:11 11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
Hebrews 4:4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works. 5 And in this place again, If they shall enter into my rest. 6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief: 7 Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts. 8 For if Jesus had given them rest, then would he not afterward have spoken of another day. 9 There remaineth therefore a rest to the people of God. 10 For he that is entered into his rest, he also hath ceased from his own works, as God did from his. 11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.

Exodus 20:12 Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee.
Ephesians 6:1 Children, obey your parents in the Lord: for this is right. 2 Honour thy father and mother; (which is the first commandment with promise;) 3 That it may be well with thee, and thou mayest live long on the earth.

Exodus 20:13 Thou shalt not kill.
Exodus 20:14 Thou shalt not commit adultery.
Exodus 20:15 Thou shalt not steal.
Exodus 20:16 Thou shalt not bear false witness against thy neighbour.
Exodus 20:17 Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's.

KJV Romans 13:9 For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.

Scholars have found gold in the grammar and syntax of even the most mundane statements in scripture.

None of it is unprofitable for doctrine and those who fear God do well immersed in it, for unlike the teachings of men, hear it.....and your soul will live:

KJV Isaiah 55:1

1 Ho, every one that thirsteth, come ye to the waters, and he that hath no money;
come ye, buy, and eat; yea, come, buy wine and milk without money and without price.

2 Wherefore do ye spend money for that which is not bread? and your labour for that which satisfieth not?
hearken diligently unto me, and eat ye that which is good, and let your soul delight itself in fatness.

3 Incline your ear, and come unto me: hear, and your soul shall live; and I will make an everlasting covenant with you, even the sure mercies of David.

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The times of this ignorance God winked at; but now commandeth all men every where to repent:

Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.-kjv