10 God asks Cain where his brother is, not as if He did not know, but as a judge asks a guilty criminal. Cain replies that he knows not, and that he is not his brother's keeper. And what answer can the Jews give at this day, when we ask them with the voice of God, that is, of the sacred Scriptures, about Christ, except that they do not know the Christ that we speak of? Cain's ignorance was pretended, and the Jews are deceived in their refusal of Christ. Moreover, they would have been in a sense keepers of Christ, if they had been willing to receive and keep the Christian faith. For the man who keeps Christ in his heart does not ask, like Cain, Am I my brother's keeper? Then God says to Cain, "What have you done? The voice of your brother's blood cries unto me from the ground." So the voice of God in the Holy Scriptures accuses the Jews. For the blood of Christ has a loud voice on the earth, when the responsive Amen of those who believe in Him comes from all nations. This is the voice of Christ's blood, because the clear voice of the faithful redeemed by His blood is the voice of the blood itself.
Source: Reply to Faustus the Manichaean (New Advent)