Just as the OT was only ever written to the children of God under the old covenant. Likewise the NT was written only to those Father draws to His Son under the new covenant:

2 Peter 1:1  Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ:

John 6:44 No man can come to me, except the Father that sent me draw him: and I will raise him up in the last day.

Peter used the word “us” in 2 Peter 1:1 to refer to himself along with any Christians who “obtained like precious faith” in Jesus Christ as our savior. Toward the end of his letter, Peter used the word “us-ward” to again include himself with fellow Christians warning that God was longsuffering with us willing that none of us should perish but come to repentance:

2 Peter 3:9  The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.

The word “repentance” means to change our mind:

https://www.blueletterbible.org/lexicon/g3341/kjv/tr/0-1/

When we are born from above as babes in Christ we have a very ignorant mind to what the scriptures teach. Therefore we must long for the milk of the Word to grow in our salvation:

1 Peter 2:1-2 Putting away therefore all wickedness, and all guile, and hypocrisies, and envies, and all evil speakings, (2) as newborn babes, long for the spiritual milk which is without guile, that ye may grow thereby unto salvation;

We must hunger and thirst for our savior’s righteousness drinking up His promises being filled with Him:

Matthew 5:6 Blessed are they that hunger and thirst after righteousness: for they shall be filled.

As we learn our Father’s promises we must then continually exercise them in faith cleansing ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of our Father:

2 Corinthians 7:1 Having therefore these promises, beloved, let us cleanse ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of God.

This brings godly sorrow unto repentance of all sin:

2 Corinthians 7:8-10 For though I made you sorry with my epistle, I do not regret it: though I did regret it (for I see that that epistle made you sorry, though but for a season), (9) I now rejoice, not that ye were made sorry, but that ye were made sorry unto repentance; for ye were made sorry after a godly sort, that ye might suffer loss by us in nothing. (10) For godly sorrow worketh repentance unto salvation, a repentance which bringeth no regret: but the sorrow of the world worketh death.

Unfortunately many Christians are content to remain babes and become dull of hearing the deeper truths of righteousness required to become fullgrown men of God:

Hebrews 5:11-14 Of whom we have many things to say, and hard of interpretation, seeing ye are become dull of hearing. (12) For when by reason of the time ye ought to be teachers, ye have need again that some one teach you the rudiments of the first principles of the oracles of God; and are become such as have need of milk, and not of solid food. (13) For every one that partaketh of milk is without experience of the word of righteousness; for he is a babe. (14) But solid food is for fullgrown men, even those who by reason of use have their senses exercised to discern good and evil.

Those Christians who truly strive for His righteousness are purifying their souls in obedience to our Father’s Word learning to fervently love the brethren as we do works worthy of repentance:

1 Peter 1:22 Seeing ye have purified your souls in your obedience to the truth unto unfeigned love of the brethren, love one another from the heart fervently:

Acts 26:19-20 Wherefore, O king Agrippa, I was not disobedient unto the heavenly vision: (20) but declared both to them of Damascus first and at Jerusalem, and throughout all the country of Judaea, and also to the Gentiles, that they should repent and turn to God, doing works worthy of repentance.

James 2:26 For as the body apart from the spirit is dead, even so faith apart from works is dead.

Peter called this the end of our salvation “even the salvation of our souls”. This comes through manifold trials from our Father that help us learn obedient repentance deep within our soul through continued faith in Him:

1 Peter 1:6-9 Wherein ye greatly rejoice, though now for a little while, if need be, ye have been put to grief in manifold trials, (7) that the proof of your faith, being more precious than gold that perisheth though it is proved by fire, may be found unto praise and glory and honor at the revelation of Jesus Christ: (8) whom not having seen ye love; on whom, though now ye see him not, yet believing, ye rejoice greatly with joy unspeakable and full of glory: (9) receiving the end of your faith, even the salvation of your souls.

Paul, Peter, and James all agree that we as “brethren” must not “err from these truths” but if we do and another brother warns us of “the error of our way” we must repent or we will experience the death of our soul:

James 5:19-20  Brethren, if any of you do err from the truth, and one convert him;  (20)  Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins.

Unlike our body, our soul can only experience spiritual death:

Matthew 10:28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.

The soul who becomes born again from a life dead in sin but then chooses to not repent of hidden sin will spiritually die again and become “trees without fruit” who are “twice dead” and reserved for darkness of hell:

Jude 1:12-13 These are they who are hidden rocks in your love-feasts when they feast with you, that without fear feed themselves; clouds without water, carried along by winds; autumn trees without fruit, twice dead, plucked up by the roots; (13) wild waves of the sea, foaming out their own shame; wandering stars, for whom the blackness of darkness hath been reserved for ever.

As Christians, we are likened to trees who God expects to mature and bear the good fruit of Christ:

Matthew 7:17-23 Even so every good tree bringeth forth good fruit; but the corrupt tree bringeth forth evil fruit. (18) A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. (19) Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. (20) Therefore by their fruits ye shall know them. (21) Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father who is in heaven. (22) Many will say to me in that day, Lord, Lord, did we not prophesy by thy name, and by thy name cast out demons, and by thy name do many mighty works? (23) And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.

Jesus used many parables to prove to us as Christians that we must bear His fruit. In John chapter 15, He compares Himself to a Vine that only the saved become the branches growing on Him but His Father will cut us right back off and burn us in the fires of hell for not producing Jesus’ fruit which is the will of our Father:

John 15:1-6  I am the true vine, and my Father is the husbandman.  (2)  Every branch in me that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit.  (3)  Now ye are clean through the word which I have spoken unto you.  (4)  Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me.  (5)  I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.  (6)  If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.

Paul also made it very clear that we as wild olive branches could be cut back off the one olive tree if we become high minded or do not continue in the goodness of our new covenant:

Romans 11:20-22  Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:  (21)  For if God spared not the natural branches, take heed lest he also spare not thee.  (22)  Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.

To be “high minded” is to lift anything on this earth above the Word of God in our mind:

Philippians 3:17-19 Brethren, be ye imitators together of me, and mark them that so walk even as ye have us for an ensample. (18) For many walk, of whom I told you often, and now tell you even weeping, that they are the enemies of the cross of Christ: (19) whose end is perdition, whose god is the belly, and whose glory is in their shame, who mind earthly things.

Paul warned the Roman church and by extension us that if we do not put to death the deeds of our body we shall die a spiritual death:

Romans 8:12-14  Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.  (13)  For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.  (14)  For as many as are led by the Spirit of God, they are the sons of God.

Living after our flesh defiles our temple where the Spirit of God dwells and will cause our Father to destroy us:

1 Corinthians 3:16-17 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you? (17) If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.

Living after our flesh is pursuing the lust of our flesh, the lust of our eyes, and having pride in our life which are the sins of the world:

1 John 2:15-17 Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. (16) For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. (17) And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.

Jesus taught the disciples that it would be better for them as believers to remove any part of their physical bodies that cause them to sin rather than to be cast into hell with their entire body:

Matthew 18:8-9 Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire. (9) And if thine eye offend thee, pluck it out, and cast it from thee: it is better for thee to enter into life with one eye, rather than having two eyes to be cast into hell fire.

For our God is no respecter of persons for He will render to every Christian according to his deeds. Eternal life to those who obey the truth or hell for those who obey unrighteousness:

Romans 2:6-11 Who will render to every man according to his deeds: (7) To them who by patient continuance in well doing seek for glory and honour and immortality, eternal life: (8) But unto them that are contentious, and do not obey the truth, but obey unrighteousness, indignation and wrath, (9) Tribulation and anguish, upon every soul of man that doeth evil, of the Jew first, and also of the Gentile; (10) But glory, honour, and peace, to every man that worketh good, to the Jew first, and also to the Gentile: (11) For there is no respect of persons with God.

We manifest our love for Jesus by loving both in word even to the least of our brethren:

Matthew 25:31-46 But when the Son of man shall come in his glory, and all the angels with him, then shall he sit on the throne of his glory: (32) and before him shall be gathered all the nations: and he shall separate them one from another, as the shepherd separateth the sheep from the goats; (33) and he shall set the sheep on his right hand, but the goats on the left. (34) Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world: (35) for I was hungry, and ye gave me to eat; I was thirsty, and ye gave me drink; I was a stranger, and ye took me in; (36) naked, and ye clothed me; I was sick, and ye visited me; I was in prison, and ye came unto me. (37) Then shall the righteous answer him, saying, Lord, when saw we thee hungry, and fed thee? or athirst, and gave thee drink? (38) And when saw we thee a stranger, and took thee in? or naked, and clothed thee? (39) And when saw we thee sick, or in prison, and came unto thee? (40) And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye did it unto one of these my brethren, even these least, ye did it unto me. (41) Then shall he say also unto them on the left hand, Depart from me, ye cursed, into the eternal fire which is prepared for the devil and his angels: (42) for I was hungry, and ye did not give me to eat; I was thirsty, and ye gave me no drink; (43) I was a stranger, and ye took me not in; naked, and ye clothed me not; sick, and in prison, and ye visited me not. (44) Then shall they also answer, saying, Lord, when saw we thee hungry, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee? (45) Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did it not unto one of these least, ye did it not unto me. (46) And these shall go away into eternal punishment: but the righteous into eternal life.