Doubting Thomas and the Nature of Trust Context! Kierkegaards mortal enemies were the ironists, romanticists, relativists. I've heard it said that K would be just as well known as Nietzsche if he had written in a more accessible language, and not Dutch -- only he actually petitioned to be able to write in Dutch, rather than the more universal and academic latin that one normally would.
This "lack of passion" is rather an attack on not really believing, and feeling things fully, and powerfully -- but rather aways lukewarm, doubtful, relativistic everything, and nothing is true bullshit, and no one really feels or believes anything completely anymore.
His message wasn't that people should sin, but rather that people should really believe, and feel things fully, become entirely invested, consumed, and act with that genuineness, or good faith. This is superior to not believing or feeling anything with conviction, and not sinning, as well as doing a whole lot of nothing else either.