John Wesley thought such perfection was a reality for some in this lifetime. It was a possibility for all, though not all took up the challenge.
When I attended a Bible college in the Wesleyan tradition, we were taught that "All men may be saved; all men may know they are saved; All men may attain unto Christian perfection". Of course, we spent hours learning what all that meant.
As an earlier post points out, the definition of sin is important. At the same school we were taught that "sin is a wilful transgression of the known law of God". It is the Wesleyan teaching of "entire sanctification" (a.k.a. Christian perfection, perfect love, etc.) that addresses the sin condition, as opposed to sinful behaviours.
I really didn't find the transition to the Orthodox understanding of theosis a big leap; in fact, since the Orthodox Church gives us the tools to deal with our sinful state much more concretely than Wesleyan/holiness churches, it was actually a factor in drawing me into the Church.