If believers can't sin, why does Satan have anything to accuse them of?
Satan has something to accuse believers of because believers are not sinless in this life; they remain subject to sin in the body of flesh and are prone to fall. If sinning were impossible, Satan would have no accusation, yet he accuses them before God until they enter the Lord's presence.
Les explains that Revelation 12:10 calls Satan "the accuser of our brethren," accusing believers before God day and night. He points out that if it were impossible for a believer to sin, Satan would have nothing to accuse them of, but he did, does, and will until we are in the Lord's presence. Believers are subjected to sin, must be constantly on guard, and remain prone to fall as long as they are in this body of flesh. A true believer does not deliberately plan to get drunk, commit adultery, or default somebody, but it can happen; when a believer sins, there is an advocate with the Father, Jesus Christ the righteous.
And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.