King Benjamin declared the natural man to be an enemy to God (see Mosiah 3:19). Ammon here describes the natural man as the man who is without repentance, faith, good works, and constant prayer. A person ceases to be a natural man when the Holy Ghost becomes his companion. The natural man is the man devoid of the spirit of prophecy and revelation.
The man of God, as contrasted with the natural man, has the promise that he may know things that are not generally known and that he may be an instrument for righteousness in the hands of God to bring many unto repentance. The promise is not appended to a priesthood office or to a particular calling. Indeed, the promise is not limited to men. It extends to every faithful member of the Church (see also D&C 76:5-10).