Commentaries:
The demon invaded the man's mind, overriding his conscious personality, which allowed the unclean spirit to speak through him (Mark 5:7). Knowing that God's Son would come in the flesh to save humanity, and that God is raising firstfruits for His Family among humanity, the demons resentfully lust for victory over people. Though God mercifully limits demon possession, He often allows demons to influence people heavily, as seen in the unclean spiritual condition of this society. Thus, Christians must beware of worldly relationships (II Corinthians 6:15-18).
Martin G. Collins
The Miracles of Jesus Christ: Exorcism in the Synagogue
Mark calls the usurping demon "unclean," indicating moral impurity. It is also a troublemaker, as seen in its contrary attitude. It is interesting that the demon violently recoils when faced with the authority of the spiritually clean Christ. Recognizing His holiness and unwilling to remain in Jesus' presence, it cries out in fear.
We see a similar uncleanness in those who trouble a congregation; they invariably live spiritually impure lives. They may talk about God's love and other virtuous things, but it often merely camouflages their evil intentions. These troublemakers are not peaceable, righteous people, but, like their spiritual father, Satan, they are of the lie (see John 8:44; II Thessalonians 2:9-12), and oppose those who truly serve Him.
Martin G. Collins
The Miracles of Jesus Christ: Exorcism in the Synagogue
Other Forerunner Commentary entries containing Mark 1:23:
Mark 8:22-26