The 'sickness' which Kierkegaard's book refers to as 'unto death' is resistance to this belief. It is the inclination to accept that as far as the individual is concerned, death is indeed the end. Now why should Kierkegaard want to call that a sickness? After all, even in his own time there must have been people strong both in might and body who rejected the Christian teaching of sin and salvation, and who faced what they accepted as total extinction...