John Cotton answers the question: Why would God ever let his people fall? For certainly God could stir the heart of a man to never lose its fervor. Yet the love of men grows cold; men walk carelessly; even the soundest believer will fall (Ecclesiastes 7:20; Genesis 12:13; Luke 22:61). What reason could have God to permit even his best servants to fail? Certainly God would receive more glory by the moral perfection of his people. Yet, yet, he lets them fall.
The capstone of Paul’s gospel presentation in Romans ends with the declaration that God transforms sin by means of mercy:
What use is there in such knowledge:
Use 3. To shew us where our happiness lieth, to wit, not in our own innocency, but in the covering of our sins, Ps. 32:1, 2; and therefore we seek for all our righteousness in Christ, Phil. 3:7–9; Rom. 3:23, 24.
John Cotton, Ecclesiastes