Cardinal Manning defines formal sin as “a sin committed with a full knowledge of what we do, and a full consent to do it.” Material sins are “the same actions done without sufficient knowledge, or without intention. Two men may commit the very same action, and the one be guilty before God, and the other not guilty. If, in the dark, I think that I am felling a tree, and with my axe I cut down a man, I am not a murderer. I have committed manslaughter in the dark, and without intention. . . . The quality of sinfulness therefore is purified, and taken away from the action, if I do not know what I am about, and if I do not intend it.”
Accordingly, Christ prayed for His crucifiers: “Father, forgive them; they know not what they do.” Cardinal Manning comments: “In His Divine compassion He prayed for His crucifiers; and the Apostle, speaking of Him, says: ‘Whom none of the princes of this world knew: for had they known Him they would not have crucified the Lord of Glory.’ That is to say, among the multitude, perhaps the greater number did not know what they did, and that Divine prayer of compassion reveals a law of God’s equity and pity upon the ignorant.”
“Nevertheless, those who know, or have it in their power to know, are guilty; for we are responsible not only for all that we do know, but for all that we might know, and therefore ought to know.”
Since the distinction between formal and material sin depends upon the knowledge and intention of the actor, Cardinal Manning points out the crucial distinction between vincible and invincible ignorance. “Ignorance takes away the guilt of our actions if that ignorance be invincible, for then we cannot overcome it. If we could not know any better, then God in His infinite mercy, though we have committed a material sin, will not take account with us as if it were a formal sin. But there is another kind of ignorance which is called vincible, because it may be overcome if we use the proper diligence to know; and God has put within our reach the means of knowledge sufficient if we will diligently seek it.”
Quotations from Henry Edward Manning, Sin and Its Consequences, 2d ed. (London: Burns and Oates, 1874).