The central message of this sermon is that salvation is a process that involves an initiator (God), transmitters (those who relay the message, including God's people), and receivers (the lost). The sermon highlights the importance of fulfilling one's role in this process, specifically the role of the transmitter, and warns against presuming on God's grace by assuming that disobedience has no consequences. The story of Jonah serves as a model for understanding God's sovereignty and initiative in salvation, emphasizing that it is God who chooses to use flawed individuals like Jonah to convey His message, and that our role is simply to obey and relay that message, trusting in God's power to bring about transformation in the receivers.