The context of the warning given by Nephi seems to be saying that without the power of the Holy Ghost, and a knowledge of the mysteries of God, the natural man is prone to do wickedly and be found unclean at “the judgment-seat of God” (vv. 20–21). If this was not his intent, it is still a true concept. King Benjamin certainly taught this doctrine:
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. [Mosiah 3:19]
Nephi had many mysteries unfolded to him and the Holy Ghost gave him authority to record them for our benefit (v. 22). A knowledge of the mysteries revealed to him are certainly needed in the world today and will help us from being cast off.