The phrase “natural man” occurs in 1 Corinthians 2:14: “But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.”
We see it again in 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.”
Both of those verses use the term “natural man” as the opposite of the “spiritual man.” The concept has nothing to do with a person’s nature, only that it is possible to exist as either one who has been touched by the Spirit, or one who has not.
In this context, the Lamanites had been “natural men,” in that they did not know God. The sons of Mosiah had been “natural men” in that they did not recognize God correctly. How then can one move from being a “natural man” to a “spiritual man”? Ammon declares “there is none that knoweth these things, save it be the penitent.” One must repent.
Note in verse 22 that Ammon does not say that one must “have” faith. Rather, one must “exercise” faith. True faith motivates us to act according to gospel principles and laws. True faith moves us to “bring forth good works.”