"... if any community on earth is bound by considerations of personal interest, to encourage the diffusion of sound religious principles among the lower orders, we are that community."

What the minister is claiming by this quote is that southern society revolves around slave labor and its effiency. The more work done, the greater the profits. Religion increases morality and at the same time productivity. He  is saying that slaves need to be inspired by morality and piety to increase productivity and in turn create higher profits.