Jamieson, Fausset and Brown on Ephesians 4:18

18. More literally, "Being darkened in their understanding," that is, their intelligence, or perceptions (compare Ep 5:8; Ac 26:18; 1Th 5:4, 5).

alienated--This and "darkened," imply that before the fall they (in the person of their first father) had been partakers of life and light: and that they had revolted from the primitive revelation (compare Ep 2:12).

life of God--that life whereby God lives in His own people: as He was the life and light in Adam before the irruption of death and darkness into human nature; and as He is the life in the regenerate (Ga 2:20). "Spiritual life in believers is kindled from the life itself of God" [Bengel].

through--rather as Greek, "on account of the ignorance," namely, of God. Wilful ignorance in the first instance, their fathers not "choosing to retain God in their knowledge." This is the beginning point of their misery (Ac 17:30; Ro 1:21, 23, 28; 1Pe 1:14).

because of--"on account of."

blindness--Greek, "hardness," literally, the hardening of the skin so as not to be sensible of touch. Hence a soul's callousness to feeling (Mr 3:5). Where there is spiritual "life" ("the life of God") there is feeling; where there is not, there is "hardness."