Can a believer still sin after being set free from sin?

Yes. A true believer can still yield to old Adam and sin, though God is not pleased. Scripture warns not to let old Adam reign in the mortal body. If a believer dabbles in sin, God disciplines; persistent chastening may lead to the sin unto death.

Les explains that both a Spirit-filled believer and a fleshly-minded believer are still believers. A true believer can therefore let old Adam reign in his mortal body and obey its lusts, but God is not pleased. When people say such a one is no longer saved, Les reminds them that God first disciplines. Hebrews says, “Whom God loveth, he chasteneth.” He chastens the believer, not the unbeliever, if the believer starts dabbling in sin. If chastening does not wake him, God may take him out—the sin unto death, which Paul teaches. Believers have freedom and free exercise of will, but obnoxious Christian behavior brings God’s spanking, and God is the originator of discipline.

Romans 6:12

Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.

How are believers admonished to live after being set free?

The admonition is to reckon themselves dead to the old Adam and alive to God through Jesus Christ, choosing to be led by the Spirit rather than live carnal, fleshly lives. Believers are to keep old Adam in subjection, live profitable lives that honor God, and not let old Adam reign in their mortal bodies.