Powerful missionary activity has always been compelled by the conviction that the light of nature is not sufficient to bring men to a right relationship with God. Men must hear the good news of Jesus Christ in order to be converted. Here is David Brainerd's testimony to that conviction.
"It is easy to harangue upon the excellency and advantage of the light of nature. It is agreeable to the pride of mankind to exalt the powers of human reason, and pronounce it a sufficient guide to eternal happiness. But let us inquire into the records of antiquity, let us consult the experience of all ages, and we shall find, that those who had no guide but the light of nature, no instructor but unassisted reason, have wandered in perpetual uncertainty, darkness, and error. Or let us take a view of the present state of those countries that have not been illuminated by the gospel; and we shall see, that notwithstanding the improvements of near six thousand years, they remain to this day covered with the grossest darkness, and abandoned to the most immoral and vicious practices."
(taken from his ordination sermon, preached June 12, 1744)