A.T. Robertson Commentary Mark 1:23

"And straightway there was in their synagogue a man with an unclean spirit; and he cried out," — Mark 1:23 (ASV)

With an unclean spirit (εν πνευματ ακαθαρτω). This use of εν "with" is common in the Septuagint like the Hebrew be, but it occurs also in the papyri. It is the same idiom as "in Christ," "in the Lord" so common with Paul. In English we speak of our being in love, in drink, in his cups, etc. The unclean spirit was in the man and the man in the unclean spirit, a man in the power of the unclean spirit. Luke has "having," the usual construction. See on Mt 22:43. Unclean spirit is used as synonymous with

demon (δαιμονιον). It is the idea of estrangement from God (Zechariah 13:2). The whole subject of demonology is difficult, but no more so than the problem of the devil. Jesus distinguishes between the man and the unclean spirit. Usually physical or mental disease accompanied the possession by demons. One wonders today if the degenerates and confirmed criminals so common now are not under the power of demons. The only cure for confirmed criminals seems to be conversion (a new heart).