4 But we have still in reserve for the further solution of this question the blessed apostle, and that robber who was a villain in wickedness, but a believer on the cross. For the Apostle Paul tells us that he was made an apostle not of men, nor by man, but by Jesus Christ: and speaking of his own gospel, he says, “For I neither received it of man, neither did I learn it, but by the revelation of Jesus Christ.” How then was he among those of whom it is said, “They shall believe in me through their word”? On the other hand, the robber believed at the very time when in the case of the teachers themselves such faith as they previously possessed had utterly failed. Not even he, therefore, believed on Christ through their word, and yet his faith was such that he confessed that He whom he saw nailed to the cross would not only rise again, but would also reign, when he said, “Remember me when You come into Your kingdom.”
Source: Tractates on the Gospel of John (New Advent)