No sort of defense is needed for preaching out of doors; but it would need very potent arguments to prove that a man had done his duty who has never preached beyond the walls of his meeting-house. A defense is required rather for services within building than for worship outside of them. Apologies are certainly wanted for architects who pile up brick and stone into the skies when there is so much need for preaching rooms among poor sinners down below.
Lectures to My Students, p 254 | Charles Spurgeon