ANSWER: The gross idolatry of Israel had been cast out through captivity in Babylon, so that they were "empty and swept" of that evil, but they had permitted a sevenfold greater to come in, namely, Pharisaism embracing self-righteousness and unbelief culminating in the rejection of their Messiah King and Savior. To show the enormity of these sins and their dreadful consequences he uses a parable suggested by his miracle just wrought by casting out a demon (verse 22). The whole description bears a parabolic impress, and the several features should not be overstrained or "made to go on all fours." The whole subject of demoniacal possessions is greatly alleviated by the idea that while Christ was on the earth the Almighty lengthened their tether, permitting them to take possession of some men in Judea, where a basement window was left open, so that Jesus by casting them out could demonstrate that demons were subject to him while angels ministered unto Him, thus showing his universal Lordship. This theory is sustained by the fact that demonical possessions did not often occur in the Old Testament nor to any great extent in the New Testament after the time of Christ. "Waterless" or desert, places are witnesses of the sin of mankind — a sticking proof of the disappearance of Paradise. Hence deserts are suitable places for demons outside of hell. See Isa. 13:21, 22; 34:13-15, Rev 18:2. The number seven denotes completeness. "Empty and swept" signify sloth; "the idle man's brain is the devil's workshop." "Garnished" — here the pure soul appears as the bride wooed by heaven and hell. The unclean man is fascinated by purity which he desires to defile. This passage of Scripture is full of deep moral truth, the most alarming of which is the possibility of a Christian's final and eternal apostasy. "Wherefore let him that thinketh he standeth take heed lest he fall."
— Steele's Answers pp. 198, 199.