The major impact of philosopher Søren Kierkegaard lies in his development of existentialism and the concept of the individual's relationship with faith and choice. Kierkegaard emphasized the necessity for individuals to confront the anxiety and absurdity of life, advocating for personal responsibility and authenticity. His works, such as *Fear and Trembling* and *Either/Or*, challenged traditional philosophical and religious frameworks, influencing later thinkers and existential writers. His focus on subjective experience and the importance of faith contributed significantly to modern philosophy and psychology.