What is the greatest sin according to the Bible?

“If I had not done among them the works which none other man did, they had not had sin,” said the Lord Jesus, as though the sin of a Cain, of an antediluvian world, of a Babel, of a Sodom and Gomorrah, of a Tyre and Sidon, of a Pharaoh, of a Jeroboam, or of a Ahab, were not to be placed in the same scale, or to be measured in the same measure; as though every previous act of transgression, disobedience, and blasphemy were now completely eclipsed. “Now,” says He, “they have no cloak for their sin.” His rejection was the sin of sins—the crowning sin, yea, the damning sin! Bad as it was to “thrust aside” a Moses, “treat shamefully” a Jeremiah, or “slay” a Zechariah “between the temple and the altar,” what shall we say of that act whereby the Son of God is disallowed and murdered?

Assuredly all men are sinners, "For all have sinned." (Rom. 3:23) I learn then that there is not one man now in the whole of this world that can either have forgiveness of sins, or be fit for the presence of the Holy God, except by the blood of the Lord Jesus, who was the sacrifice once offered for sins. Oh, what swarms of despisers and rejecters of Christ in this day. Art thou one, my reader? Perhaps thou sayest, "I am not so great a sinner as many; I try to pay all I owe; and do no harm to any one." Hold! do no harm to any one! Hast thou believed in Jesus, to the saving of thy soul? Dost thou say, No? Then art thou the greatest of all sinners; for to reject Christ is the greatest of all possible sins, and this thou wilt find to thy cost for ever. To disbelieve Jesus is to sin against God, who sent him to die for sinners. To sin against that precious Christ, who died for sinners. To sin against thyself; oh, sin of all sins; to plunge thyself into eternal torment. Is this doing no harm? This is that monster sin, of which the Holy Ghost convinces; "Of sin, because they believe not on me." (John 16:9) Ah! ponder this well. Thou hast but a moment to live compared with eternity; and if thou shouldst continue to reject the pardon preached through the blood of Jesus, then, when thou liftest up thine eyes in torment, then wilt thou know what thou hast done in rejecting Christ.

"Of sin because they believe not on Me." Not because they have done this, that, and the other thing, they ought not to have done. All this, however true, falls short of the most serious short-coming. For Christ is the touch-stone, and they had not "loved the Highest when they saw" Him. Nay, they had both seen and hated both Him and His Father, But how the majesty of our Lord stands out! and how inconceivable that any should so speak who was merely man! Being what He was, and is, the language is not only intelligible, but the spiritual mind is conscious that nothing short of it could meet the case. "Talk they of morals, O Thou bleeding Love? The grand morality is love of Thee." So wrote long ago a poet little read now-a-days, too serious probably for a sadly flippant age. But he had evidently grasped the great truth that the want of belief in, and of love to Christ, "lies at the root of all sin and reveals its nature." All is summed up in surrender to the Holy One of God.

Of all sins, beware of the rock of unbelief. "Take heed lest there be in any of you an evil heart of unbelief" (Heb. 3:12). Men think, as long as they are not drunkards or swearers, it is no great matter to be unbelievers. This is the gospel sin. It disparages Christ's infinite merit as if it would not save; it makes the wound of sin to be broader than the plaster of Christ's blood. This is high contempt offered to Christ, and is a deeper spear than that which the Jews thrust into His side.

Of all sins this is the worst. For it makes God a liar if we believe Him not (1 John 5:10). Unbelief is the only sin which, if persisted in, is unto eternal death. The difference between a real Christian and an unreal one is, that the former does not draw back unto perdition, but believes to the saving of the soul (Heb. 10:39). Have I then a heart for Christ? Can I trust the word of God, who cannot lie, more implicitly than I can rely on my heart, which is deceitful above all things? Can you not do this, reader? Let not the word which may be trembling to escape your lips be still repressed because you yet feel the sense of personal vileness. God is not addressing you because you are not vile, but because you are so. He preaches peace to them who in themselves can have no real peace. He bestows His own righteousness in Christ in the same word which pronounces the full forgiveness of your sins if you believe in Jesus. The depth of our own feelings in regard to sin is in one sense immaterial. It is not by an inference that we are justified, but by faith. Conclusions inferred from within are exactly opposite to the venture of faith, that casts all on Jesus at the bidding of God's word.

Man has crucified "the Lord of glory" (1 Cor. 2:8). That is, they have given the place of the greatest shame and disgrace, to the One to whom all glory belongs, the one in whom "all the fullness of the Godhead dwells bodily" (Col. 2:9). Man has rejected the promised Messiah, the King, – the only one who could bring peace, prosperity, and justice. This can only lead to the judgment by God (Mt 21:33-46). Man has rejected the Son, the full revelation of the Father. This sin eclipses everything else: "If I had not come and spoken to them, they had no sin; but now they have no excuse for their sin" (John 15:22, cf. v. 24).

Take the most wonderful example of all. The greatest crime this world has seen, or can see, is the murder of the Son of God. One, who only displayed love, whose life was spent in undoing the works of the devil, was done to death by the insensate hate of the religious leaders of the nation.

Here we have His present relation, not to the saints, but to that world which is outside. And the Lord tells us that, when come, He ἐλέγξει the world. It is difficult to convey justly the force of this. "Reprove," as in the Authorised Version, is too narrow a meaning, if not false. "Rebuke" is here out of the question. "Convict" hardly applies, even to the first, not at all to the second and third clauses; and supposes an effect produced which may not really be in any case. Nor is one satisfied with "convince," save in the sense of affording proof by His presence, rather than by His action. For by His coming and abiding in the saints, apart from the world, He gives it demonstrative proof of sin, of righteousness, and of judgment.

The law dealt with Israel as those under it. But now it is the Spirit Who demonstrates the "sin" of the world; and this not because they violate that Divine measure of a man's duty, but because they reject the Son of God: "of sin, because they believe not on Me." He had come in grace; to reject this was fatal. It is not merely failure in obligation, but despite of God's love. Such is the true and actual gauge of the world before God, Who tests and proves the guilt of the whole system which opposes Him by its unbelieving ignorance and refusal of His Son, spite of the fullest testimony. This is the sin demonstrated.297

He, coming, shall convince the world about sin, etc. This is His office. He does not speak of effectual result, though of course there was such. About sin, indeed, in this, that "they believe not on me." It was not merely sinfulness or transgressions; both might have been charged naturally, and by the law; but their very state, their whole state with God. The former, sunk under the bright coming of Jesus from God the Father, were forgotten in this abounding act of love. This proved the manner of God's dealings with the world; not then, that is, in the presenting of Jesus to it. God was in Christ reconciling, and not imputing. They saw no beauty in it or Him. He was despised and rejected of men. They esteemed Him not. Thus was the accumulated evidence of their utter and desperate alienation of mind brought out. They were sin, and nothing else; for where all beauty was they saw none; where God was manifested in the flesh He was not known; when He came to His own in the way of personal grace they received Him not, but hated and rejected Him. This is the great argument, the argument of sin to the world. Here is the ground on which it stands before God, howsoever besides it may have transgressed, as indeed they had. They believe not on Jesus; that is the whole matter, the summing up of all, the concentrating and exhibition of all; enmity against God most displayed in grace; enmity to death; the slave of Satan. Here sin was triumphant, most utterly so, as far as it could be in its weakness and destructiveness in the death of Christ the Lord. But not believing was the same thing as to position.

"He will convince the world of sin, … because they believe not on me." It is not here as Messiah to the Jews that the Lord speaks of Himself, but as the Son of God to the world, as such. It was "sin" not to know the Father nor Him. The charge here is not that of having killed the prophets or broken the law; but "they believe not on me." God had sent His Son into the world, and He had been cast out. (He says this in view of its accomplishment.) The very presence of the Holy Ghost stamps the world with this sin. He could not be sent here, unless Jesus had been rejected - unless God's own Son had been cast out. He had wrought always: this is His personal mission and presence on earth.