It is St. Paul who wrote the doctrine about hair. His teachings came from our Lord Jesus Christ...
Gal 1:11 But I certify you, brethren, that the gospel which was preached of me is not after man.
Gal 1:12 For I neither received it of man, neither was I taught it, BUT BY THE REVELATION OF JESUS CHRIST.
The doctrine about hair is not only for MEN. It includes also all Christian women. If a woman have long hair it is a glory to her.
1Co 11:14 Doth not even nature itself teach you, that, if a man have long hair, it is a shame unto him?
1Co 11:15 But if a woman have long hair, it is a glory to her: for her hair is given her for a covering.
The teaching of Saint Paul about the hair starts here...
1Co 11:3 But I would have you know, that the HEAD of every man is Christ; and the HEAD of the woman is the man; and the HEAD of Christ is God.
Here it says that the HEAD of Christ is GOD (The Almighty Father), then Christ is the HEAD of MAN and MAN is the HEAD of the WOMAN. In Christian faith, the highest in hierarchy is God The Father Almighty, then his Son who is our Lord Jesus Christ, then the Man and the Woman.
1Co 11:4 Every man praying or prophesying, having his head covered, dishonoureth his head.
If a man is praying or preaching he must not cover his head because he dishonored his HEAD which is Jesus Christ. Cover his head means having long hair because long hair is a covering.
1Co 11:15 But if a woman HAVE LONG HAIR, it is a glory to her: for her HAIR IS GIVEN HER FOR A COVERING.
1Co 11:5 But every woman that prayeth or prophesieth with her head uncovered dishonoureth her head: for that is even all one as if she were shaven.
On the other hand, if a woman is praying with head uncovered or short hair she dishonors her HEAD or her husband.
1Co 11:7 For a man indeed ought not to cover his head, forasmuch as he is the image and glory of God: but the woman is the glory of the man.
A MAN should not cover his HEAD (Jesus Christ) because he is the image and glory of God.
1Co 11:10 For this cause ought the woman to have power on her head because of the angels.
A Christian Woman must have a sign on her head because of the Angels. In the former times the Angels (Sons of God) saw that the daughters of men were beautiful and they took them as wives. It produces Giants on earth as a result of crossbreeding between Angels and Men. Those Giants became mighty men and so the Earth became corrupt before the Great Flood.
Gen 6:1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
Gen 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
Gen 6:3 And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
Gen 6:4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Then I believe this thread starts here which you can easily understand why Christians Men must have short hair.
1Co 11:13 Judge in yourselves: is it comely that a woman pray unto God uncovered?
The rightful answer is NO!
1Co 11:14 Doth not even nature itself teach you, that, if a man have long hair, it is a shame unto him?
The rightful answer is YES!
1Co 11:15 But if a woman have long hair, it is a glory to her: for her hair is given her for a covering.
For Christians, it is very important that the doctrine of hair is followed. It is not God's will that Christian Men sport long hair and on the other hand it is God's will that Christian women have long hair. Therefore our Lord Jesus Christ doest not have long hair because he is the one who teaches Saint Paul about the HAIR.