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Sin is anything that is not love. Love does not even think an evil thought. Love will not sin.
Genesis' depiction of the fall places ownership of sin's manifested state on the serpent. He carried the disease of "doubt". Regardless of our interpretation, God's cold blooded reptiles were made to wear this fate, according to Book 1. Unbelief, selfishness, evil...whatever you want to name "sin" to be, the commonality is that sin is what we chose/choose instead of God. If we read satan to be the serpent, and former glorified Lucifer, then we must link sin's inception to an event in the spiritual realm either at the moment of creation, or directly after. But...where is the clear link showing showing this Lucifer>serpent>satan evolution. Certainly, at man's fall the serpent was manifest to a different state.
Go through the gospels and research "faith". Count how many times Jesus explains that "faith" has healed, or made well, or moved mountains. Look at how many times He tells His disciples that they are doubting way too much...Thomas, for example. God asked Adam and Eve for faith; the Lord Jesus asks the same. I agree that "self" is the enemy of faith and God, thereby making it sin. I contend that this is sin word that passed through the serpent to man. From God comes the word of life : FAITH; from the serpent comes the word of death: SIN.
Just some thoughts. I follow Jesus Christ...author of my life.