1 Corinthians 6:9-11 says nothing about becoming a Christian, then going back to living in sin and becoming unrighteous all over again. That is your eisegesis. It's actually the opposite. The apostle Paul mentions the unrighteous and describes their sinful lifestyles then goes on to say in verse 11 - "And such were (past tense) some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God."
In Romans 8:30, we read - Moreover whom He predestined, these He also called; whom He called, these He also justified; and whom He justified, these He also glorified. *ALL of them. *Notice how Paul uses the past tense for a future event to stress its certainty.
You suffer from acute case of
Cherry Pickers Syndrome causing you to reject God's warnings to Christians clearly indicating it's possible for them to fall way and not go to Heaven if they don't confes and turn away from their sin.
Your claim is people can go ahead and sin and God just plays like they didn't which is a doctrine of demons.... to hold your OSAS position you have to believe Jesus lied in John 15 where speaking of believers He said if any man does not abide IN Him and bares good fruit.. he is cut off.
Here's another one the OSAS peoples cannot explain way:
1 Corinthians 9:27
I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
Castaway (G0096 in Strong's) = unapproved, rejected, worthless, reprobate.
This is the same greek word used in Romans 1:28 when the Lord turned those over to a reprobate mind who refused retain God in their knowledge and walk with Him.
This means, Paul believed that if he did not control himself and went back to walking after the flesh
HE would be unapproved, rejected, worthless, reprobate!
Obviously the Apostle Paul did not believe in "once saved, always saved" aka "eternal security"
Here's another one that the OSAS peoples cannot understand due to their blindness:
Hebrews 10:26
For if we sin willfully after that we have received the knowledge of the truth, there remains no more sacrifice for sins
*If we knowing and willing continue in sin, Jesus’ sacrifice on the Cross is no longer applicable and there will be no new sacrifice for sin forthcoming other than what Jesus has already done at the Cross. One must cease from sin for the work of Christ to be applicable to them.