16 You ought not to say, in disparagement of Moses, that he was a sinner, and that he was put to death on a mountain because his God was angry with him. For Moses could glory in the Lord as his Saviour, who is also the Saviour of him who says, "Christ Jesus came into the world to save sinners, of whom I am chief." Moses, indeed, is accused by the voice of God, because his faith showed signs of weakness when he was commanded to draw water out of the rock. In this he may have sinned as Peter did, when from the weakness of his faith he became afraid in the midst of the waves. But we cannot think from this, that he who, as the Gospel tells us, was counted worthy to be present with the Lord along with holy Elias on the mount of transfiguration, was separated from the eternal fellowship of the saints.
The sacred history shows in what favor he was with God even after his sin. But since you may ask why God speaks of this sin as deserving the punishment of death, and as I have promised to point out prophecies of Christ in those passages which you select for criticism, I will try, with the Lord's help, to show that what you object to in the death of Moses is, when rightly understood, prophetical of Christ.
Source: Reply to Faustus the Manichaean (New Advent)