Everlastingly
Part of speech: adverb
Definition: In an everlasting manner; so as to be everlasting.
Eternally
Part of speech: adverb
Definition: For eternity; forever.
Example sentence: People who cease to believe in God or goodness altogether still believe in the devil. I don't know why. No, I do indeed know why. Evil is always possible. And goodness is eternally difficult.