“Having the understanding darkened, being alienated from the life of God through the ignorance that is in them, because of the blindness of their heart:”
King James Version (KJV)
4:18 Having their understanding darkened, through the ignorance that is in them - So that they are totally void of the light of God, neither have they any knowledge of his will. Being alienated from the life of God - Utter strangers to the divine, the spiritual life. Through the hardness of their hearts - Callous and senseless. And where there is no sense, there can be no life.
Eph 4:18 Having the understanding darkened. He shows how the Gentiles walk in vanity. They are in darkness, unenlightened by the gospel. Alienated from the life of God. Without the spiritual life that comes from knowing God. Because of blindness of their heart. The idea is a willful blindness.