God formed Adam and Eve with his own hands. And, he communed with them (they recognized His voice after their sin and hid in the garden, which suggests that He had been there before with them, talking face to face). That is perfect harmony, before sin. How does perfect harmony lead to sin? You often accuse the "once saved always saved" group of sinning all they want. The operative word here is "want." Wanting to sin is sin. (Mat 5:28) What about the person that no longer wants to sin but still sins, like Paul? (Romans 7:15) As long as it is not willful sin, regular sin is fine.
Gen 6:5 And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Does this include Adam, the first man? We see Adam's actions. God sees his heart. The condition of Adam's heart did not change when he ate the fruit, the condition was revealed. He was naked before and naked after. I don't know if Satan, the accuser, told him that he was naked but he is surely Satan would be on the short list of suspects to answer God's first question "Who told thee that thou wast naked?" Then their eyes were opened and the light shined in. Whatsoever doth make manifest is light.
Sin begins in the heart. Eve gives a hint when she says it is a tree to be desired to make one wise. We are not told how or when sin entered the world through one man. It may be that sin is inherent in the creation of the will. It may be that Satan's sin did not count as part of the world. It may be part of the mystery that continues which will be finished at the seventh trumpet. (Rev. 10:7)