Whoso belongs only to his own age, and reverences only its gilt Popinjays or soot-smeared Mumbojumbos, must needs die with it.

Carlyle contrasts a life rooted only in contemporary fashion with one nourished by enduring ideals. The “gilt Popinjays” suggest flashy, shallow idols of polite society; the “soot-smeared Mumbojumbos” evoke grimy, superstitious or mechanized false gods—two different but equally hollow objects of reverence. To “belong only to his own age” is to let one’s mind be bounded by the transient opinions, institutions, and celebrities of the moment. Carlyle’s warning is that such a person perishes spiritually and intellectually when the age passes, because he has not attached himself to permanent truths, great minds, or moral realities that outlast any single historical period.