In his remarkable book, The Suffering Savior, Frederick Krummacher made the astonishing observation that by Jesus' prayer for mercy from the cross for those who crucified him, we can conclude that "while offering up the Lord Jesus, [those crying out for his death] unconsciously pay the ransom for themselves, and thereby render it possible for God to have mercy upon them, without detracting from his justice." He wrote:
"We now fully comprehend the tone and perfect certainty, firmness, and confidence with which the words, "Father, forgive them!" are uttered. The High Priest pronounces them from the most holy place, and that too at the very moment when he is paying the debt of the guilty. That he really does this, and that the true meaning of his sufferings is to be sought in this, he once for all evinces to a sinful world from his elevation on the cross; and hence, while bleeding on their behalf, he sends up to heaven this unconditional petition for mercy in favor of the vilest sinners, his murderers.
But how could the Lord commend these hardened rebels to divine mercy? Observe, my friends, that those whom he had in view, were by no means hardened. For such as have committed the "sin unto death" there is certainly no longer any deliverance or salvation, and according to the apostle's directions, we ought not to pray for such. But the Lord well knows what he is doing. Although he says at first, "Forgive them," which is certainly very general, yet he immediately limits his words, so that Judas, for instance, and doubtless many of the heads of the people, are excluded from the influence of his intercession. The addition of the words, "They know not what they do," defines its bounds. By this clause the Lord selects from the multitude which surrounds him those to whom the majority of them that crucified him probably belonged. They were not like the Pharisees, who accused Jesus of casting out devils by Beelzebub, and had therefore committed the sin against the Holy Ghost, but they were under a delusion, which was certainly far from guiltless, when they consigned Jesus to death.
Now observe, first, the sublime self-possession which the Lord here again manifests in the words, "They know not what they do." For what other meaning lies concealed beneath them than this, that if they had known it was the Lord of Glory, or even some innocent and just person, they would not have done it? For in the words, "They know not what they do," the idea is included that while offering up the Lord Jesus, they unconsciously pay the ransom for themselves, and thereby render it possible for God to have mercy upon them, without detracting from his justice.
Finally, the words, "They know not what they do," must be apprehended in the same sense in which I must be understood, if I likewise said of any one whom I had come to deliver out of his distress, but who, ignorant of my intention, basely repulsed me, "He knows not what he is doing." In this case, my meaning would be, "Have patience; he will soon recollect himself when he is aware who I am, and for what purpose I entered his abode, and will then act differently toward me." I thus utter a prediction, and such a one is doubtless included in our Lord's words. They contain a veiled prediction of the future repentance and conversion of those for whom he prays. For even by this petition a powerful impulse to repentance is given them, and a direction to a change of mind. Only look forward a little, and you will already see, first, in the Roman centurion under the cross, and his shield-bearer, the commencement of the fulfillment of that prediction. Mark, then, the crowds who, returning from Calvary to Jerusalem, smote upon their breasts, and, at least in part, gave evidence of sincere repentance. Assuredly among them were some to whom the petition, "Father, forgive them," applied. But if they were not among these, they were decidedly among the three thousand who were pierced to the heart by the apostles' words on the day of Pentecost. For listen to the address of Peter: "This Jesus," says he, "whom ye have crucified, hath God made both Lord and Christ. Now, when they heard this," the narrative states, "they were pricked in their hearts, and said, Men and brethren, what shall we do?" Yes, it was these who knew not what they did, but now it became evident to them. O how did the remembrance of the words, "Father, forgive them," smite humblingly and overwhelmingly upon their hearts! How did the love which was manifested in those words melt their souls! Alas! alas! they had nailed to the cross their only Deliverer and Savior! Could it be otherwise than that under such reflections their eyes became fountains of tears? But the repentance for which the consolation of forgiveness first made room in their souls, issued in devotedness to the Lord, and in their being faithful to him even unto death. Thus did the petition, "Father, forgive them, they know not what they do," neither overthrow the statutes of divine justice, nor the method of grace, once for all established by the Lord. Justice retained its splendor, by virtue of the satisfaction of the only-begotten Son, and the plan of salvation was preserved entire in the repentance and conversion of them to whom the petition applied."1
Frederick Krummacher The Suffering Servant (Boston: Gould and Lincoln, 1859) pp. 386-387