Comprehend what I wrote. The word SINCE is not employed in this verse since Paul is not addressing unbelievers. That is why he used the word IF because only believers can choose IF to live by the flesh or IF to live by the Spirit. Therefore v.13 can in no way apply to unbelievers or false brethren. If Paul were addressing unbelievers in the first clause of this verse, he would have employed SINCE but he did not do so. Therefore, Paul's intention is clear. He warns the brethren in Rome that if they habitually sin by living according to the flesh, they will die; i.e. spiritual death.
Which Greek word that means "since" would Paul have used in Rom 5:18?
You are still ignoring vs. 14 and the implication which I have stated twice.
Indeed, everyone in the church is not an obedient believer hence his warning of spiritual death for those who chronically sin which evidences an unrepentant lifestyle. A believer can choose to be disobedient. An unbeliever has no such choice regarding disobedience.
Joshua seemed to think the unrepentant could make a choice.
They are not sons of God because instead of being led by the Spirit, the are led by the flesh - the result of which is spiritual death. We are talking about one class of people, i.e. believers only as I explained above. If a believer lives according to the flesh, he faces the prospect of spiritual death. If a believer lives according to the Spirit, he will spiritually live. If you are implying that these people are really unbelievers then I reject that for my reasons stated above.
You are free to reject anything you want to but as I stated before Paul's audience included both.
John 15:7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
Jesus evidently thought His followers could stop abiding in Him.
Mark 11:26 But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses.
Jesus evidently thought His followers could fail to forgive and thus not be forgiven.
John 8:31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
Jesus evidently thought His believers could fail to continue in His word and thus not be disciples.
1Co 15:1-2 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;1 Corinthians
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
Paul evidently thought the Corinthians could forget the gospel and not be saved.
Colossians 1:23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
Paul evidently thought the Colossians could fail to continue in the faith and move away from the gospel.
1 Thessalonians 3:8 For now we live, if ye stand fast in the Lord.
Paul evidently thought the Thessalonians could not stand fast and thus not live.
Hebrews 3:14
(14) For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
The writer of Hebrews evidently thought it possible for his audience to fail to hold on to their confidence and be steadfast to the end.
Hebrews 6:4-6
(4) For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,
(5) And have tasted the good word of God, and the powers of the world to come,
(6) If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
The writer of Hebrews evidently thought it was possible for believers to fall away.
The only impossibility stated is being restored if they fall away. Only believers can fall away.
You need not mention it. I mentioned it because correct me if I'm wrong but you believe in eternal security. Rom 8:13 counters that belief that a genuine believer can never lose their secure position in Christ. If Jesus taught that a Christian is not secure in his salvation then the principle of first mention requires that all scripture written by the Apostles be interpreted in light of what Jesus taught.
See above.