Do You Believe? Really Believe—That Christ is the Son of the Living God?
“The Sin of Unbelief” 
(Taken from the writings of John Albert Broadus, founder of The Southern Baptist Theological Seminary. Broadus’s writings are in the Public Domain.)
We are in God’s world—we are bound to receive his teachings as truth, to rest upon his promises, and obey his precepts.
But it is unbelief with respect to the Son of God, which is denounced in the text and elsewhere as a flagrant sin. In the text, the dignity of Christ’s character seems presented as magnifying [the] guilt of unbelief— “the only begotten Son of God.” Consider Jesus as the mediator, the offered object of faith, in condescension to our infirmity—consider too his love, sympathy, invitations—and then estimate the sin of rejecting Christ.
This removes all possibility of questions as to one being a sinner— “but now they have on cloak for their sin.” Men often seek to cover up their sin beneath the cloak of various pretenses and shadowy, vain excuses. But not question about this sin, whether they realized its guilt or not. This of itself is sufficient to condemn!! Would you know your standing before God? There is no need to argue with others, extenuating and excusing—the text settles the question. Suppose the catalogue of your sins were read to decide your character before God. Whenever this sin is reached, “he hath not believed,” . . . then and there the examination will cease, the question is decided.
Already, without examining further, the man is condemned.
This is true, dear friends who are unconverted, of you all. As Peter on the Day of Pentecost spoke to the people, so would I to you. He did not stop to accuse them of particular sins, nor to consider how much merit there might be in particular excellencies, he did not speak of all that terrible wickedness which then so much abounded, not of Pharisaic pride nor of Sadducean skepticism—he dwelt upon their rejection of Jesus, the Son of God, both Lord and Christ. It was the consciousness of the crowning sin that pricked their heart, and made them cry. . .. I do not stop to speak of vices, nor even of general alienation of heart—I solemnly say, what God’s Word declares, [that] you are condemned as unbelievers in Christ. We may recognize your personal worth in many respects, but you have been rejecting Christ. Often his salvation had been offered, and you have refused to accept it. Do not say you are not an avowed infidel—without that, one may be guilty of unbelief—without that, the Scriptures declare you are guilty.
But someone may say, how can unbelief be a sin (though this Scriptures declares it), when I cannot help it? I am unable to believe. The Saviour [sic] said, “No man can come unto me.” Ah, my friend, do not deceive yourself by that specious excuse.
The Scriptures also said, “Ye will not come,”—are not willing. If a man is unable to believe, it is only because he is unwilling to believe. Inability is not like that to fly to the stars, not to know the future—not to lack capacity, but unwillingness. And does this diminish guilt? The more opposed a man is to doing his duty, the more he is blameworthy. It a servant neglects a plain duty, doe his lack of inclination exculpates him? The more averse is one’s heart to Christ, the more unwilling he is to believe, so much the greater must be the guiltiness of unbelief. No, no, you are verily guilty.
But, why [would I make] this argument and appeal [to you]? Why, might someone say, does a man who wishes others to be happy, labor to convince them that they are very sinful, condemned to destruction? Why should it be said of the Comforter, that he would convince the world of sin? Is there any other comfort in such a conviction? Is it not more pleasant, is it not wiser, to forget sin and judgment, that to be reminded of it? The man who feels it true of himself that he is condemned already may know that other gracious truth that “there is no condemnation.” “He that believeth is not condemned.” That very sin of unbelief, which seals your condemnation, may suggest the way of pardon. Cease to reject Jesus, receive him as Savior. Acknowledging [your] guilt; pray for mercy, through him. Jesus is able to save you!! Will you ask him to save you, and continue to ask? Oh, that you would!!
 Vernon Latrelle Stanfield, Favorite Sermons of John A. Broadus (New York: Harper & Brothers Publishers, 1959), 136-138.