What is the meaning of the phrase, "Where thirsting longing eyes / Watch the slow door / That opening, letting in, lets out no more" in Echo?

The meaning of the phrase, "Where thirsting longing eyes / Watch the slow door / That opening, letting in, lets out no more," is that in Paradise, love, once begun, never dissipates; once lovers are admitted into a mutual paradise, their love never evaporates, and is never dissolved.