I hadn't realized these two verse go together until just now. I knew that 2 Nephi 9 answered the "what if" question "What if there had been no ateonement?" The answer given there is that all mankind would become devils and angels to a devil for all eternity.
Exactly how that works isn't spelled out. Is it saying there would be no such thing as repentance without the atonement?
But that scripture needs to be paired with the words of an angel:
Mosiah 3:19 For the natural man is an enemy to God, and has been from the fall of Adam, and will be, forever and ever, unless he yields to the enticings of the Holy Spirit, and putteth off the natural man and becometh a saint through the atonement of Christ the Lord, and becometh as a child, submissive, meek, humble, patient, full of love, willing to submit to all things which the Lord seeth fit to inflict upon him, even as a child doth submit to his father.
The first line of Mosiah 3:19 is not just an angel using exaggeration for effect. He is saying that the natural man is an enemy to god, and will be forever and ever unless he becomes a saint through the atonement. His statement that the natural man will be an enemy to God forever and ever is a statement about each individual .Each individual would be an enemy to God forever and ever unless they yield to the enticings of the Holy Ghost and put off the natural man and become a saint through the atonement of Christ.
This is the same point, more or less, that is being made in 2 Nephi 9 when we read that
2 Nephi 9:8 O the wisdom of God, his mercy and grace! For behold, if the flesh should rise no more our spirits must become subject to that angel who fell from before the presence of the Eternal God, and became the devil, to rise no more.
9 And our spirits must have become like unto him, and we become devils, angels to a devil, to be shut out from the presence of our God, and to remain with the father of lies, in misery, like unto himself; ...
The power Satan gains over us through the weakness that is in the flesh would have no end if not for the atonement. His power over us would continue in the next life. We would be under his sway, and under his influence. We would remain shut off from the gift of the Holy Ghost, because there would be no atonement that could redeem us from our sins and make us worthy receptacles of the divine spirit. Satan gains influence over us as we yield to the desires of the flesh in one way or another. Our body's inclinations are not, by nature, subjected to God's will. The adversary uses our body to tempt us and get us under his power and influence. And if we are to pass on in that state, what do we find? He still has power over us. We would even recognize his familiar whispers in the spirit world and would continue to follow him. It goes beyond that. It is not just the influence of a known advisor, or the influence of familiarity. He also gains real power and influence, and that would continue with us into the next life.
Having no way to end that, we would continue to be, forever and ever, angels to a devil, and become devils ourselves. That is what is being conveyed in both of theses scriptures.