Cyril of Jerusalem

The chief author of sin is the devil, the begetter of all sin. Before him, no one sinned. Nor did he sin because he was by nature prone to sin (since in that case the responsibility for his sin would lie with his Creator). Rather, being created good he became a devil by his own free choice, receiving that name from his willed action. Though he was originally an archangel, he became a slanderer (diabolos), because of his slandering.