Would the tendency to sin come from being of the flesh, earthly? The sin nature is what we inherited as a result of Adam's sin. There is a good read on it here: https://www.gotquestions.org/sin-nature.html
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They define the sin nature as:
"When we speak of the sin nature, we refer to the fact that we have a natural inclination to sin; given the choice to do God’s will or our own, we will naturally choose to do our own thing."
But about the origin of it they say:
"By that one action [Eve´ disobedience], sin entered into their nature."
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Which does not make sense, because she chose to do her own thing before the act.