John Calvin Commentary Acts 5:15

"insomuch that they even carried out the sick into the streets, and laid them on beds and couches, that, as Peter came by, at the least his shadow might overshadow some one of them." — Acts 5:15 (ASV)

The shadow of Peter, as he came. The Papists abuse this text as a pretext, not only to commend feigned miracles, which they claim are done at the graves of martyrs, but also to boast of their relics. Why (they say) should not the grave, or garment, or the touching of the bones of Peter have power to heal, just as his shadow had this power?

I answer, we must not immediately think that what Luke says was done by ignorant men and those who did not know the pure faith, is right. Yet we have a more certain answer ready than this. For the apostles were endowed with such power because they were ministers of the gospel.

Therefore, they used this gift insofar as it served to further the credit of the gospel; indeed, God did no less show forth his power in their shadow than in their mouth. Those miracles about which the Papists babble are so unlike these that they are, rather, altogether contrary. For this is the purpose of their miracles: to lead the world away from Christ to saints.