That's a man made tradition, it's not found anywhere in the Bible. All true believers have the Holy Spirit, but not all true believers are ordained to teach. .
30. The lack of a definitive teaching authority in Protestant (as with the Catholic magisterium) makes many individual Protestants think that they have a direct line to God, notwithstanding all of Christian Tradition and the history of biblical exegesis (a “Bible, Holy Spirit and me” mentality). Such people are generally under-educated theologically, unteachable, lack humility, and have no business making presumed “infallible” statements about the nature of Christianity.
150 Reasons Why I Became (and Remain) a Catholic
There is the relationship: the Scriptures are a tool for "teaching, for reproof, for correction, and for training in righteousness," but who is to use this tool for these purposes? That is, who has the authority to teach, reproof, correct, and train others in righteousness? The "man of God" has this authority.
But, who is the "man of God?" Ferris Beuller may wish to claim this title for himself as well, but a short survey of Scripture's use of the title will reveal that this, too, is a privileged title that cannot be simply taken upon oneself:
Moses - "This is the blessing with which Moses the
man of God blessed the children of Israel before his death." (Deut. 33:1)
"Then the people of Judah came to Joshua at Gilgal; and Caleb the son of Jephunneh the Kenizzite said to him, 'You know what the LORD said to Moses the
man of God in Kadesh-barnea concerning you and me.'" (Josh. 14:6)
The Angel of the Lord - "Then the woman came and told her husband, 'A
man of God came to me, and his countenance was like the countenance of the angel of God, very terrible; I did not ask him whence he was, and he did not tell me his name...' Then Manoah entreated the LORD, and said, 'O, LORD, I pray thee, let the man of God whom thou didst send come again to us, and teach us what we are to do with the boy that will be born.'" (Jud. 13:6, 8)
Samuel - "The servant answered Saul again, 'Here, I have with me the fourth part of a shekel of silver, and I will give it to the
man of God, to tell us our way.'" (1 Sam. 9:8)
Elijah - "And she said to Elijah, 'What have you against me, O
man of God? You have come to me to bring my sin to remembrance, and to cause the death of my son!'" (1 Kings 17:18)
Elisha - "And she went up and laid him on the bed of the
man of God, and shut the door upon him, and went out... When Elisha came into the house, he saw the child lying dead on his bed." (2 Kings 4:21, 32)
David - "According to the ordinance of David his father, he appointed the divisions of the priests for their service, and the Levites for their offices of praise and ministry before the priests as the duty of each day required, and the gatekeepers in their divisions for the several gates; for so David
the man of God had commanded." (2 Chr. 8:14)
St. Timothy - "But as for you,
man of God, shun all this; aim at righteousness, godliness, faith, love, steadfastness, gentleness." (1 Tim. 6:11)
Contrary to the opinion that the "man of God" can be any Christian without distinction, Scripture itself will not allow such an interpretation, insisting that the "man of God" is a figure of authority, either commissioned by God directly through Divine Intervention (such as Moses or the Angel), or appointed by another holder of authority (such as Samuel, David, Elisha, and St. Timothy).
From this very brief survey of the phrase "man of God" (there are perhaps a dozen or so more passages...), we see that what holds true for "pastors" holds true for the "man of God": it is a title of authority that can in no way be taken upon oneself, but rather, it is bestowed upon a man by a higher authority. A man must be called by God to hold this title of "man of God."
By What Authority - A Challenge to Protestant Pastors