Mormon declares that even though they are losing the spiritual battle for the hearts of the Nephite people, they must continue to try to call them to repentance. That is their mission, and they are accountable for fulfilling their mission, even if the mission is not successful. Agency is powerfully operative in this earth, and when we exercise our agency in the context of the gospel, we benefit from those choices. Other people may exercise their agency to deny the gospel, and they reap the “benefits” of that rejection. However, in the economy of God, their rejection of our message does not diminish the growth we receive from delivering the message.
One of the important aspects of this letter is that Mormon uses the collective term “let us.” He might be speaking more generically, but it would appear that this letter comes after Moroni’s call to the ministry, and that this really is a mutual admonition to effort. Both Mormon and Moroni are in positions that require this effort of them, and Mormon is emphasizing their mutual responsibility to preach repentance.