Existentialism is unusual among philosophical schools in refusing to settle its own central religious question, and the refusal is close to the point. Soren Kierkegaard, writing in Denmark a century before the word existentialism existed, is now read as the movement's founding religious voice. He argued that Christian faith is not a conclusion reason can reach by argument but a leap made in the face of the absurd, undertaken by a single individual standing alone before God with no crowd, no system and no proof to hide behind. Fear and Trembling reads the biblical Abraham not as an edifying example but as a man asked to do something ethics itself cannot justify, and Kierkegaard's authorship circles back to that same demand: faith costs exactly what makes it faith rather than certainty.
A century later Jean-Paul Sartre framed the same starting point toward the opposite conclusion. If there is no God, Sartre argued, there is no fixed human nature handed down in advance the way a manufactured object's purpose precedes its making, so a person exists first and only becomes something through the choices that follow. Sartre called this being condemned to be free, since not choosing is itself a choice and no one else can be blamed for the outcome. Both lines share Kierkegaard's original diagnosis, that existence is concrete, first person and anxious, and that abstract theorizing about human nature in general fails to meet it. What Kierkegaard's leap affirms, Sartre's freedom is left to carry alone, and existentialism has never resolved which is the more honest response, because resolving it was never its project.