Being or becoming a New Creation in Christ does not mean sinlessness. Paul Himself expressed the life long struggle with sin as:
“I find then a law, that evil is present with me, the one who wills to do good. For I delight in the law of God according to the inward man. But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members. O wretched man that I am! Who will deliver me from this body of death? I thank God—through Jesus Christ our Lord!” (Romans 7:21-25)
What then is the difference between sin in the old creation not IN CHRIST, and the new creation which is IN CHRIST?
It is like the difference between the alcoholic and the social drinker (not recommended however as they are the ones that become alcoholic according to Upton Sinclair in CUP OF FURY**), a matter of either ocassional sin or habitual as in a specific habit of sin.
The difference in the Bible is expressed in I John with the difference between “sin” (which if anyone says that he does not is a liar and the truth is not in him) or “sinneth not” which is the caught in continuous year after year and time after time habitual of the same kind sin, and without any repentance and in confession of said sin and forgiveness of God.
You do recognize hopefully that the devil is a constantly or habitual sinner, of such it is recorded in I John 3:8.
“He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.” (I John 3:8 KJV)
So the “committeth sin” is the habitual without repentance and forgiveness, or even confessed sin; or the same with “sinneth from the beginning”. Clear, hopefully.
**In CUP OF FURY, Upton Sinclair tells of numerous famous literary friends like Jack London, Sinclair Lewis, Ernest Hemingway, and on and one who intended to be social drinkers only; then in every one of their lives the drinking got out of hand as they became alcoholics and had ruined lives and died directly as a result of the demon alcohol.