Why is Paul annoyed (16:18)? He apparently was worried that people might associate the demonically possessed slave girl with the gospel, and not differentiate between the demonic and the power of God! So, he frees her by casting out the demon, and her owners get mad because she can’t make them money anymore. Paul and Silas end up getting beaten and arrested, which we find out later is against the law. A Roman citizen isn’t allowed to be beaten or imprisoned without a formal hearing. Some may think that’s the reason for their singing – they knew they were going to be released! However, they also trust God and know that He is working, and God uses their imprisonment to bring the jailer and his family to faith. Can you think of a time in your own life when God has taken difficult circumstances and used them for the purpose of furthering his kingdom?
Two observations: (1) Acts 12:19 reveals that the death penalty was often used for escaped prisoners, which is why the soldier was so ready to take his own life, and (2) although some see 16:31 as an indication that the head of the house can believe for his family, 16:32 shows that the message was preached to the rest of the household as well. This seems to indicate that belief is personal.