4 sentences

1. 由于山路崎岖，书信很难达于外界。

   yóu​yú shān​lù qí​qū, shū​xìn hěn nán dá yú wài​jiè.

   Because of the rugged mountain roads, it's difficult for letters to reach the outside world.

   Source: Dong Chinese Dictionary

2. 他们两家多年来一直有书信往来。

   tā​men liǎng jiā duō nián lái yī​zhí yǒu shū​xìn wǎng​lái.

   The two families have corresponded by letter for many years.

   Source: Dong Chinese Dictionary

3. 在古代，书信是人们远距离交流的主要方式。

   zài gǔ​dài, shū​xìn shì rén​men yuǎn​jù​lí jiāo​liú de zhǔ​yào fāng​shì.

   In ancient times, letters were the main way for people to communicate over long distances.

   Source: Dong Chinese Dictionary

4. 圣保罗的书信对基督教神学产生了深远影响。

   Shèng​bǎo​luó de shū​xìn duì Jī​dū​jiào shén​xué chǎn​shēng le shēn​yuǎn yǐng​xiǎng.

   The epistles of Saint Paul have had a profound impact on Christian theology.

   Source: Dong Chinese Dictionary