'He in whom the five beings and the ether rest, him alone I believe to be the Self,—I who know, believe him to be Brahman; I who am immortal, believe him to be immortal.

In plain words: The speaker says that the five kinds of beings and space itself all rest in this One. He sees it as his true Self and as Brahman. Because he knows it, he is immortal, just as it is immortal.