The first occurrence of this expression is in Deut. 33:1, and is used of Moses. Its use in connection with Moses (Ps. 90, title), who was, par excellence, the prophet, like unto whom Christ was to be "raised up" (Deut. 18:15-19), shows that it is to be understood of what Moses was, viz., "the prophet". He was so called, not because he foretold, but because he spoke FOR God. This is the meaning of the word "prophet" as taught by its first occurrence in Gen. 20:7. The prophet was God's "spokesman" (Ex. 4:16. Cp. Ex. 7:1).
God's spokesman could know what to speak for Him only (1) from His Spirit (Neh. 9:30. Cp. Hos. 9:7, margin, and see Num. 11:16, 17, 25-29); (2) from Jehovah making Himself known (Num. 12:6. Ezek. 3:17. Jer. 15:19. Cp. 2Chron. 36:12); and (3) from God's written word. This is why Timothy is the only one called a "man of God" in the New Testament (1Tim. 6:11), and why, to-day, one, and only one who knows "all scripture", which is so profitable, can be called a "man of God" (2Tim. 3:17).
All such are God's spokesmen because they alone know what He wishes to be spoken. They are His witnesses (Acts 1:8; 22:15). Christ was THE prophet because He spoke only those things which were give Him to speak (see note on Deut. 18:18), and He alone is "the faithful Witness" (Rev. 1:5).
It was for the above reasons that the expression "the man of God" (i.e. God's man) became the general name for a prophet among the common people. See all the occurrences :--