Jamieson, Fausset and Brown on John 3:31-34

31-34. He that, &c.--Here is the reason why He must increase while all human teachers must decrease. The Master "cometh from above"--descending from His proper element, the region of those "heavenly things" which He came to reveal, and so, although mingling with men and things on the earth, is not "of the earth," either in Person or Word. The servants, on the contrary, springing of earth, are of the earth, and their testimony, even though divine in authority, partakes necessarily of their own earthiness. (So strongly did the Baptist feel this contrast that the last clause just repeats the first). It is impossible for a sharper line of distinction to be drawn between Christ and all human teachers, even when divinely commissioned and speaking by the power of the Holy Ghost. And who does not perceive it? The words of prophets and apostles are undeniable and most precious truth; but in the words of Christ we hear a voice as from the excellent Glory, the Eternal Word making Himself heard in our own flesh.