And suddenly - While they were praying and singing.
A great earthquake - Matthew 28:2. An earthquake, in such circumstances, was regarded as a symbol of the presence of God, and as an answer to prayer. See the notes on Acts 4:31. The design of this was, doubtless, to furnish them proof of the presence and protection of God, and to provide a way for them to escape. It was one among the series of wonders by which the gospel was established, and the early Christians protected amidst their dangers.
And immediately all the doors were opened - An effect that would naturally follow from the violent concussion of the earthquake. Compare Acts 5:19.
Everyone’s bands were loosed - This was evidently a miracle. Some have supposed that their chains were dissolved by electric fluid; but the narrative gives no account of any such fluid, even supposing such an effect to be possible. It was evidently a direct interposition of divine power. But for what purpose it was done is not recorded. Grotius supposes that it was that they might know that the apostles might be useful to them and to others, and that by them their spiritual bonds might be loosed. Probably the design was to impress all the prisoners with the conviction of the presence and power of God, and thus to prepare them to receive the message of life from the lips of his servants Paul and Silas. They had just before heard them singing and praying; they were aware, doubtless, of the cause for which they were imprisoned; they saw evident tokens that they were the servants of the Most High, and under his protection; and their own minds were impressed and awed by the terrors of the earthquake, and by the fact of their own liberation. It renders this scene the more remarkable, that though the doors were opened, and the prisoners loosed, yet no one made any attempt to escape.