D.L. Moody emphasizes that unbelief is the root of all sin, labeling it as the 'climax sin' of the world. He argues that many societal issues stem from this fundamental lack of faith, which leads individuals to live in sin and rebellion against God. Moody calls for the Holy Spirit's convicting power to reveal the seriousness of unbelief, stressing that true conversion must be accompanied by a deep conviction of sin. He warns against superficial conversions that lack genuine repentance and urges believers to pray for a thorough work of conviction in the hearts of men. Ultimately, Moody highlights the Spirit's role in imparting life and guiding believers into truth while convicting the world of sin.

That is the sin of the world. Why, a great many people think that unbelief is a sort of misfortune, but do not know, if you will allow me the expression, it is the damning sin of the world today; that is what unbelief is, the mother of all sin. There would not be a drunkard walking the streets, if it were not for unbelief; there would not be a harlot walking the streets, if it were not for unbelief; there would not be a murderer, if it was not for unbelief, it is the germ of all sin. Don’t think for a moment that it is a misfortune, but just bear in mind it is an awful sin, and may the Holy Spirit convict every reader that unbelief is making God a liar. Many a man has been knocked down on the streets because some one has told him he was a liar. Unbelief is giving God the lie; that is the plain English of it. Some people seem to boast of their unbelief; they seem to think it is quite respectable to be an infidel and doubt God’s Word, and they will vainly boast and say, "I have intellectual difficulties; I can’t believe." Oh that the Spirit of God may come and convict men of sin! That is what we need -His convicting power, and I am so thankful that God has not put that into our hands. We have not to convict men; if we had I would get discouraged, and give up preaching, and go back to business within the next forty-eight hours. It is my work to preach and hold up the Cross and testify of Christ; but it is His work to convict men of sin and lead them to Christ. One thing I have noticed, that some conversions don’t amount to anything; that if a man professes to be converted without conviction of sin, he is one of those stony-ground hearers who don’t bring forth much fruit. The first little wave of persecution, the first breath of opposition, and the man is back in the world again. Let us pray, dear Christian reader, that God may carry on a deep and thorough work, that men may be convicted of sin so that they can not rest in unbelief. Let us pray God it may be a thorough work in the land. I would a great deal rather see a hundred men thoroughly converted, truly born of God, than to see a thousand professed conversions where the Spirit of God has not convicted of sin. Don’t let us cry "Peace, peace, when there is no peace." Don’t go to the man who is living in sin, and tell him all he has to do is to stand right up and profess, without any hatred for sin. Let us ask God first to show every man the plague of his own heart, that the Spirit, may convict them of sin. Then will the work in our hands be real, and deep, and abide the fiery trial which will try every man’s labor. Thus far, we have found the work of the Spirit is to impart life, to implant hope, to give liberty, to testify of Christ, to guide us into all truth, to teach us all things, to comfort the believers, and to convict the world of sin.