Why do you suppose Paul has to tell Christians not to let sin reign over them? Should they not already be perfect in every way possible?
The Sanctification Process takes time for some believers. So no. Not everyone is a cookie cutter clone in doing good overnight. For some, it takes time to put away their grievous sin. It's why we are told to work our salvation with fear and trembling (Philippians 2:12). It's why we are told to cleanse ourselves from all filthiness of the flesh and spirit perfecting holiness in the fear of God (2 Corinthians 7:1). For others it can be an almost overnight process or within a very short time. I believe this to be the case for Enoch, and the 144,000. It depends on the heart of the individual.
Anyways, you ask: "Why do you suppose Paul has to tell Christians not to let sin reign over them?"
Paul answers this by saying:
"Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness? " (Romans 6:16).
Jesus says he that sins is a "servant of sin," but the servant will not abide in the house forever (See: John 8:34-35). This means that the believer who commits grievous unrepentant sin (or who justifies sin) will be cast out of the house of the Son of God. How so? We see an example of this in Matthew 13:41-42. It says:
The Son of Man (JESUS) will send forth his angels and they will gather out of HIS KINGDOM, all things that offend (i.e. those who make others to sin), and all who work iniquity (i.e. those who work sin or lawlessness), and they (the angels) will cast them into the furnace of fire (i.e. the Lake of Fire).
John says he that commits sin is of the devil (1 John 3:8).
John says everyone who does evil hates the light (John 3:20).
The light is Jesus Christ.
Can a person hate Jesus and be saved?
Jesus says if you love me, keep my commandments (John 14:15).
Paul says if any man loves not the Lord Jesus Christ, let him be accursed (1 Corinthians 16:22).