Jamieson, Fausset and Brown on Ephesians 1:18

18. understanding--The oldest manuscripts, versions, and Fathers, read "heart." Compare the contrary state of unbelieving, the heart being in fault (Ep 4:18; Mt 13:15). Translate, "Having the eyes of your heart enlightened" (Ep 5:14; Mt 4:16). The first effect of the Spirit moving in the new creation, as in the original physical creation (Ge 1:3; 2Co 4:6). So Theophilus to Autolycus (1.3), "the ears of the heart." Where spiritual light is, there is life (Joh 1:4). The heart is "the core of life" [Harless], and the fountain of the thoughts; whence "the heart" in Scripture includes the mind, as well as the inclination. Its "eye," or inward vision, both receives and contemplates the light (Mt 6:22, 23). The eye is the symbol of intelligence (Eze 1:18).

the hope of his calling--the hope appertaining to His having called you; or, to the calling wherewith He has called you.

and--omitted in the oldest manuscripts and versions.

riches of the glory--(Col 1:27).

his inheritance in the saints--The inheritance which he has in store in the case of the saints. I prefer explaining, "The inheritance which He has in his saints." (See on Ep 1:11; De 32:9).