Paul and John both thought it was possible to sin and yet still be a born-again believer:
1 Corinthians 3:1-4; 6
In regards to 1 Corinthians 3:
Well, just because they are called brethren, does not mean they are saved brethren. In 1 Corinthians 5 we learn that we are to cast out that wicked brother from among us (who is committing sexual immorality within the gathering body of believers).
11 "But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolator, or a railer, or a drunkard, or an extortioner; with such an one no not to eat.
12 For what have I to do to judge them also that are without? do not ye judge them that are within?
13 But them that are without God judgeth. Therefore put away from among yourselves that wicked person." (1 Corinthians 5:11-13).
Paul calls them carnal in 1 Corinthians 3.
6 "For to be carnally minded
is death; but to be spiritually minded
is life and peace.
7 Because the carnal mind
is enmity against God: for it is not subject to the law of God, neither indeed can be.
8 So then they that are in the flesh cannot please God." (Romans 8:6-8).
"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." (Romans 8:13).
Peter talks about children who are accursed.
"Having eyes full of adultery, and that cannot cease from sin; beguiling unstable souls: an heart they have exercised with covetous practices; cursed children:" (2 Peter 2:14).
I have heard the Eternal Security Proponent boast about how they cannot cease from sin more times than I care to count. Why would Peter describe a trait to false propehts if it is the same trait as believers?
1 Corinthians 3 is talking about your motivations behind your good works. Are they works solely based upon the Lord or a little bit so as to please men? Works that are not solely done out of love for the Lord will be burned up. Those who defile their temple by committing grievous sin will be destroyed.
James 5:19-20 refers to the reader as brethren and if anyone of us brethren errs away from the truth and is converted back again (by another faithful believer), they should know that they have helped to save a soul from death and help to cover their sins (no doubt because they got them to repent of their sins to the Lord for living a prodigal life of sin).
In the Parable of the Prodigal Son we learn that a brother can be dead and be made alive again (See Luke 15:32).
You said:
Paul is not telling the brethren to come together for the worse as in reference to how he is not concerned with their sin. Paul is not saying to just get together (despite their sinfulness). If that was the case, then Paul would contradict himself in many places. Paul says if any man does not consent to the words of Jesus Christ and the doctrine according to godliness, they are proud and they know nothing (1 Timothy 6:3-4). Paul told the brethren to kick out that brother who committed sexual immorality among them in 1 Corinthians 5. Try reading 1 Corinthians 11:17-31 in the NLT instead of the KJV. It will help to clear things up a bit. They are both saying the same thing but the KJV can be read in the wrong way; Especially if a person is looking to read it that way.
You said:
The OSAS interpretation on 1 John 1:8 does not work because it would contradict a normal reading on 1 John 2:3-4. This verse is a warning to the brethren about a false belief about those who seduce them (1 John 2:26). They were gnostics and they falsely believed that sin was an illusion or it did not exist. So it is saying that if a believer were to sin, ignoring it is not the way (1 John 1:8), but confessing it is the way (1 John 1:9). This is in view of John saying to "sin not." (1 John 2:1). This lines up with Jesus telling people to "sin no more" (John 5:14) (John 8:11).
You said:
Galatians 5:16 is something you are ignoring here.
It says we are able to not fulfill the lusts of the flesh if we walk in the Spirit.
This verse does not work in your belief because you no doubt take the OSAS interpretation on 1 John 1:8 that says you will always be in sin of some kind. So you are not able to not fulfill the lusts of the flesh by walking in the Spirit as Galatians 5:16 says. That verse does not exist for you or it is a verse you have to change into saying something else.
Also, Galatians 5:14 says to love your neighbor as yourself. This is a salvation issue if you do not love your neighbor. Loving God and loving your neighbor is a part of inheriting everlasting life. Please carefully read very slowly Luke 10:25-28.