It is not improbable, that what is added, to the end of the chapter, are the words of the evangelist, not the Baptist. He that is of the earth — A mere man; of earthly original, has a spirit and speech answerable to it.
Related Commentaries of John 3:31
John 3:31
31 He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all.