In Genesis 1:26-27, God created Adam in His own image and likeness. But by Genesis 5:3, Seth was born in the image and likeness of Adam. Does that mean Adam was no longer in the image of God? No, for we find in Genesis 9:6, that God warns man not to kill, saying, "Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made He man." That means Adam was still in God's image, because his offspring continue to be in God's image (Gen. 9:6). Adam ceased to be in God's likeness. His offspring, including you and me, according to these verses, are in the likeness of Adam, and not the likeness of God (Gen. 5:3; 9:6).
So how are we unlike God? We are not righteous (Rom. 3:10); we don't think like God (Isa. 55:8-9); God is Spirit; man is carnal (Rom. 8:6-7); man had a beginning, but God is eternal (1 Tim. 1:17); etc. As for our being in God's image, what does that mean? Both God and man are a trinity: God is Father, Son, and Holy Spirit (Matt. 28:19; 1 Tim. 3:16; Acts 5:1-4); man is body, soul, and spirit (1 Thes. 5:23).
Now we know that man dies as the result of Adam's sin. Paul wrote, "For since by man came death, by man came also the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive" (1 Cor. 15:21-22). Since Adam's death occurred the day he ate of the forbidden fruit (Gen. 3:5), and we know he did not die physically until hundreds of years later (Gen. 5:5), Adam must have died spiritually. That is another way man is not in God's likeness; man is spiritually dead, while God is an eternal life-giving Spirit (1 Jn. 5:11, 13, 20).
Jesus told Nicodemus that he needed to be born again, to be born of the Spirit (Jn. 3:1-8). Nicodemus obviously had been born physically, but he, like Adam, was spiritually dead. Jesus referred to the physical birth as being of water, and spiritual birth being the work of the Holy Spirit. John wrote, "Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit" (Jn. 3:5-6). Some have suggested the water refers to baptism, but that cannot be in that baptism is performed after one has been born again spiritually.
Because man is born spiritually dead (no longer in the likeness of God) as a result of the original sin committed by Adam, theologians have labeled it the Doctrine of Original Sin. Fortunately for man, Jesus has provided man with a guaranteed resurrection from the dead (1 Cor. 15:22). Unfortunately, not all will remain "raised" to life (Jn. 5:28-29). Those who were not born again will be raised "unto the resurrection of damnation," also know as the "second death" (Rev. 2:1; 20:6, 14; 21:8).
To be continued, Lord willing!