John Calvin Commentary Acts 16:26

"and suddenly there was a great earthquake, so that the foundations of the prison-house were shaken: and immediately all the doors were opened, and every one`s bands were loosed." — Acts 16:26 (ASV)

There was an earthquake. The Lord, in showing this visible sign, chiefly intended to provide for His servants, so that they might more clearly know that their prayers were heard; yet He also had regard for the others. He could have loosened the fetters of Paul and Silas without an earthquake, and also opened the gates.

But that addition served significantly to confirm them, since the Lord, for their sakes, shook both the air and the earth. Furthermore, it was necessary that the keeper of the prison and the others should feel the presence of God, so that they would not think the miracle happened by chance.

Nor is it to be doubted that the Lord then showed a sign of His power, which would be beneficial for all ages, so that the faithful may be fully assured that He will be near them whenever they are to enter conflicts and dangers for the defense of the gospel.

Nevertheless, He does not always follow the same course to testify to His presence by visible signs; nor is it permissible for us to prescribe a law for Him. For He helped His servants by visible miracles then for this reason: so that we may be content with His hidden grace today. Concerning this matter, we have spoken more in the second chapter.