The great benefit of open-air preaching is that we get so many newcomers to hear the gospel who otherwise would never hear it. The gospel command is, "Go ye into all the world and preach the gospel to every creature," but it is so little obeyed that one would imagine that it ran thus, "Go into your own place of worship and preach the gospel to the few creatures who will come inside."
Lectures to My Students, p 255 | Charles Spurgeon