Paul was arrested and detained at Philippi because he cast out the spirit of soothsaying from a slave girl. This caused an uproar among the owners of the slave girl who relied on her fortune-telling abilities to make a profit. They accused Paul and his companion Silas of disturbing the peace and advocating illegal practices, which led to their arrest and detention. Despite the circumstances of his arrest, Paul never stopped proclaiming Jesus as Lord and Saviour and used his time in prison to continue spreading the message of the gospel to those around him.