Who are the works of the flesh manifested in, believers or unbelievers?
The works of the flesh are manifest in both believers and unbelievers. Paul deals mostly with the unbeliever and his lifestyle, but a believer in a moment of weakness can also let the old Adam take over, making the works of old Adam obvious.
Les explains that the works of old Adam are manifest and obvious whether in the life of a believer who in a moment of weakness lets old Adam take over, or in the unbeliever and his lifestyle, which Paul mostly addresses. The world is saturated with immorality and wickedness because that's all they have—the old Adamic nature. Even some are inhibited by background and training, but they still cannot get victory over that old Adam without the power of God. Examples include Esau, a fornicator destitute of faith, and Ishmael, who lacked the unction to obey God and married an Egyptian contrary to God's instruction for Abraham's offspring. Esau married Canaanite women already steeped in immorality. Paul often lists sexual sins first in his classifications of human sins.