Jamieson, Fausset and Brown on Ephesians 2:16

16. Translate, "might altogether reconcile them both in one body (the Church, Col 3:15) unto God through His cross." The Greek for "reconcile" (apocatalaxe), found only here and in Col 1:20, expresses not only a return to favor with one (catallage), but so to lay aside enmity that complete amity follows; to pass from enmity to complete reconciliation [Tittmann].

slain the enmity--namely, that had been between man and God; and so that between Jew and Gentile which had resulted from it. By His being slain, He slew it (compare He 2:14).

thereby--Greek, "therein"; "in" or "by the cross," that is, His crucifixion (Col 2:15).