The theological problem raised by the book of 1 John (that the Christian is at the same time capable and incapable of sinning) is probably best solved by (a) recognizing that John is addressing two different groups within the church -- one espousing a heretical (Gnostic) type of perfectionism, and another espousing an orthodox type -- and (b) recognizing that he is presenting sinlessness as a Christian ideal but not a realized fact. Bede's affirmation is helpful here:
"In quantum in eo manet, in tantum non peccat."
I might translate this as:
"To the degree that he abides in him (i.e., Christ), to that extent he does not sin."
But the whole tenor of John's first letter cries out: "Do not sin!" When I was in seminary, I had a professor who explained it like this:
Ante casum = posse non peccare.
Post casum = non posse non peccare.
Post Christum = non posse peccare.
Today I might tweak the third line to read:
In Christo non posse peccare.
Don't you love Latin?
P. S. Bede the Venerable (d. May 25, 735) was an Anglo-Saxon historian who is best known for his Ecclesiastical History of the English People. He is also well known for his many epigrams like the one I cited above.