Just as the OT was only ever written to those 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:
The word “us” in 2 Peter 1:1 was referring to Peter and the other apostles who “obtained like precious faith” with the believers he was addressing. Toward the end of his letter, Peter again included himself with the use of the word “us-ward” as a fellow believer with them that have obtained like precious faith. He was encouraging them that Father was longsuffering to “us”-ward not willing that any of us believers should spiritually perish from our relationship with Christ:
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.
James agrees with Peter when he uses the term “brethren” that he is only speaking to the saved. If any brother “err from the truth” but does not repent after being shown “the error of his way” that brother in Christ will experience the death of his 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 ever experience a 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.
Jesus also made it very clear as the vine that only the saved become the branches growing off of Him but His Father will cut off and burn the ones who do not produce His Son’s fruit:
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 get 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.
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 cut off their hand or foot or pluck out their own eye if any part of their bodies cause them to offend God rather than to be cast into hell with all their body parts:
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 whether Christian or not for He will render to every man according to his deeds. Eternal life to those who by patient continuance in well doing seek for glory and honour and immortality but indignation and wrath unto them that are contentious, and do not obey the truth, but 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.