John Calvin Commentary Acts 4:31

"And when they had prayed, the place was shaken wherein they were gathered together; and they were all filled with the Holy Spirit, and they spake the word of God with boldness." — Acts 4:31 (ASV)

And when they had prayed. Luke declares now that God not only heard this prayer but also confirmed it by a visible sign from heaven. For the shaking of the place would, by itself, have done them little good; but it tends to another purpose: that the faithful may know that God is present with them. Ultimately, it is nothing other than a token of God's presence.

But the fruit follows, for they are all filled with the Holy Ghost and endowed with greater boldness. We should rather focus on this second part.

For while God declared His power then by shaking the place, that was a rare and extraordinary thing; and while it appeared by the effect that the apostles obtained what they desired, this is a perpetual benefit of prayer, which is also set before us as an example.