Pure of heart = naitivy = yes
Pure of heart = idealism = naivity
Kierkegaard would probably call bad faith, being untrue to one's self.
But anyway, yeah, can we really achieve the existentialist ideal (Absurd Hero, Knight of Faith, Ubermensch, etc); without being true to one's self all the way? I personally don't think so.
...and thus Kierkegaards bad faith is "the sickness unto death"...
Speaking of the existentialist ideal, is it not just an act of bad faith as it demands that you believe you belive in it?*
The ideal according to Sartre, IS the being-in-itself-for-itself which is a super-mega-ultra-impossible-being (hence man is a useless passion and a true self (to be in touch with) does no exist).
* which is how Sartre describes it in Sketch for a theory of emotions and in the chapter about bad faith in B&N)
Don't be shocked. Sartre was influenced by all three, especially Heidegger (Being and NOTHINGNESS, Being and TIME) and, after the war, by Marx (1960:Critique de la raison dialectique).
Most existential philosophers posit an ideal man that embodies their version of existential philosophy, and is something to which strive for. Camus had his Absurd Hero, Kierkegaard had the Knight of Faith, Nietzsche had the Ubermensch, Unamuno had the Immortal Man; while Dostoevsky, Kafka, and Sartre create characters in their novels and plays that embody both the strengths and weaknesses of their philosophies.
Maybe some people can and do reach this ideal, but what if most of us could only dream about achieving such ideals, never reaching it. We just go on stuck in our desparing, anguished, untermensched, bad faithed self. Does that make us failures if we do not reach an existential ideal? Is the existential ideal just too high?
Or do you think Kierkegaard is right, that just becoming and wanting to be the self you truly are, is sufficient enough? In other words, some will become great, while some will never be, but as long as you become what you want to be, that would be enough?
Everyone is perfect or ideal and will reach it in their own time and at their own pace. Finding it is accomplished by letting go of everything that is not truly you. The body, desires, judgments, thoughts, feelings, experiences etc., For some they accomplish it in 100's of lifetimes others 1000's. When you are ready to return to The Source" of your consciousness you will undo all you are not and become your perfect self.