The carnal mind is the nature in which a person's conduct is based until God acts to convert or transform him; it is man's deceitful heart (Jeremiah 17:9). Once an individual is called, and the Father and Son have revealed Themselves and some of Their purpose to him, this verse succinctly describes the major impediment to our submitting to Them. This resisting influence from within each of us is the major barrier to perfect deference and compliance to Them.
Of course, Satan and the world also influence us, but the major impediment to our responsibly submitting is what is already part of our characters even as we are being converted. We quickly revert to carnality when confronted with something that we do not want to do.
What element in our carnality drives our resistance? Solomon states in Ecclesiastes 1:2, "Vanity of vanities, all is vanity." Vanity implies something that is useless and impermanent, like vapor rising from a pot of boiling water, and therefore something of little or no value toward accomplishing God's purpose for mankind. The "all" in Solomon's statement includes us.
Indeed, You have made my days as handbreadths, and my age is as nothing before You; certainly every man at his best state is but vapor. Selah. Surely every man walks about like a shadow; surely they busy themselves in vain; he heaps up riches, and does not know who will gather them.
In Psalm 62:9, he adds, "Surely men of low degree are a vapor, men of high degree are a lie; if they are weighed in the balances, they are altogether lighter than vapor."
These are blunt statements, showing that unless something is done to change the value of what we are in reality, what good reason does God have to work with us?
But there is more from God's Word that paints the picture of our unconverted value and the strength of our natural resistance to Him even more acutely. The aforementioned Jeremiah 17:9 says, "The heart is deceitful above all things, and desperately wicked; who can know it?" "Above all things" implies all things considered evil. This by itself is a vivid comparison—and God does not lie—but He goes beyond that by adding that man's heart is not merely wicked but desperately wicked. This means our heart is without care for danger and recklessly, badly, extremely, furiously, impetuously wicked.
Jesus adds force to this word-picture by confirming in Matthew 15:17-20 that the heart is the place from which our evil resistance to God is generated. However, an irony comes into play because the heart is the same place that generates to us in our thoughts the belief that we are really something good! This is quite an effective combination in producing sin. It occurs because our hearts produce self-esteem with the result that our ideas and actions—our very lives—are focused on self-satisfaction. To meet that need, we will sin as a way of life.