One has only to look at the opening lines of a majority of his poems to see him in a state of uncertainty, mystery, doubt—that is, fear:

"When I have fears that I may cease to be”
“Glory and loveliness have Pass’d away”
“My spirit is too weak—mortality”
“O thou whose face hath felt the Winter’s wind”
“In a drear-nighted December”
“Deep in the shady sadness of a vale”
“If by dull rhymes our English must be chain’d”
“O what can ail thee, knight at arms”
“Why did I laugh tonight?
“My heart aches, and a drowsy numbness pains”

Rilke might respond to Keats:

Does Time, as it passes, really destroy?
It may rip the fortress from its rock;
but can this heart, that belongs to God,
be torn from Him by circumstance? 

Are we as fearfully fragile
as fate would have us believe?
Can we ever be severed
from childhood's deep promise? 

Ah, the knowledge of impermanence
that haunts our days
is their very fragrance. 

We in our striving think we should last forever,
but could we be used by the Divine
if we were not ephemeral?