The answer is: Theft

Josh 7:1

Achan's sin is a classic tale of individual transgression impacting the entire community, pointing to a fundamentally interconnected understanding of morality in the biblical narrative. He was part of the Israelite army, victorious after crossing into the Promised Land, yet marked by greed, he disobeyed divine instruction not to take any spoils from the city of Jericho (something meant to honor God and serve the greater good).

When he stole a beautiful Babylonian robe, silver, and gold, it reveals an essential truth: sometimes, a single person's choice can evoke a ripple effect, leading to significant consequences for others. When Achan's theft was uncovered, it led to an arduous moment for Israel—not only were they defeated in their next battle at Ai, but it also forced them to confront the deep repercussions of one man's actions on their collective fate.

Interestingly, the famed Babylonian robe Achan glimpsed at reminds us of other biblical echoes. In the wonderful world of Scripture, those who adorn in extravagant garments often symbolize wealth and pride, such as Joseph’s colorful coat, which birthed familial strife.

Achan’s story serves as ever-relevant wisdom: integrity in our actions enriches both individual and community well-being. It invites readers to explore deeper themes of collective responsibility, embered by the truths embedded in communal failure and forgiveness, illustrating the importance of following a higher code for the benefit of all.