Adam (and Chava) sinned by eating from the Tree of Knowledge. The Talmud (Eiruvin 18b) says in the name of Rabbi Meir that “Adam, the first man, was very pious. When he saw that death was imposed as a punishment because of him, he observed a fast for a hundred thirty years, and he separated from his wife for a hundred thirty years, and wore belts of fig leaves on his body as his only garment for a hundred thirty years.”
Teshuvah is the Foundation of the World
Rabbi Shlomo Kluger explains (Chochmat HaTorah, Berieshit pg. 483) that since Adam, who was hand-made by G-d, sinned, it is clear that mankind as a whole is destined to sin (from time to time). In addition, because Adam sinned and began the teshuvah process soon after he was created, this indicates that the existence of the world was founded on the ability of mankind to overcome their shortcomings and rectify them.
