If all else perished, and he remained, I should still continue to be; and if all else remained, and he were annihilated, the universe would turn to a mighty stranger.

This quote means that if everything else in the universe vanished, and only one person remained, they would still be important to the speaker. Likewise, if everything else stayed the same but that one person were to disappear, the universe would feel like a totally different and unfamiliar place to the speaker.