John Calvin on Acts 10:17-19

17. Peter was taught not only by the vision, but also by the Word of God; and yet in seeing he saw not, until the Spirit is unto him an interpreter; a most excellent mirror of our slackness. Although we be yet far unlike to Peter; for we are so far from understanding by and by what God will, or to what end he speaketh to us, that many interpretations are scarce sufficient for us. But we must also note that which Luke addeth, that Peter did think earnestly upon the vision, to wit, after that he was come to himself again after his amazedness; for this was a token of godly reverence, that he did not carelessly suffer the vision to escape him. Therefore the Lord opened to him when he did knock, (Matthew 7:7.) And we are justly plagued for our sluggishness, in that we profit no better in the Word of the Lord, seeing we are so cold, and have so small desire to inquire.