He might have suffered them to lay violent hands upon themselves, that they might not fall into the hands of their persecutors. Therefore if He neither commanded nor allowed this mode of departure from this world to His own, for whom He Himself had promised that He would prepare an eternal mansion; whatever instances may be brought by the Gentiles who know not God, it is clear that this is not lawful for those who believe one true God.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: City of God, book 1, ch. 22