6 Thirdly, we must not overlook an eventuality, even though at the present time it is improbable. Such critical conditions could arise in Europe or Asia that would compel foreign priests to leave the Indies. In that event, how could religion survive without a native clergy: with no one to administer the sacraments, no one to teach the law of God? The history of the Chinese, Japanese, and Ethiopians speaks clearly enough about this hazard. More than once, the Christians in Japan and China had to suffer from hatred and calamity. The hostile people in power exiled and executed alien priests, but spared the native. Because they were completely familiar with the language and customs of their native land and were sustained by friends and relatives, they were allowed not only to remain unharmed in their country, but also to administer the sacraments freely, and perform pastoral duties in all the provinces. On the contrary, the Ethiopia, where Christians already numbered 200,000, a sudden storm of persecution completely destroyed the fruits of long labor because there was no native clergy when the European missionaries were killed or expelled.
Source: Ad Extremas (Vatican.va)