She had doubts about his faithfulness.

她怀疑他的忠诚。

《牛津词典》

In fact, it was not the brick that made the man successful, but his faithfulness in doing even a very little thing.

事实上，使这个人成功的并不是那块砖，而是他在做哪怕是一件很小的事情时的忠诚。

youdao

His first betrothed heard of this, and fretted so much about his faithfulness that she nearly died.

他的第一个未婚妻听说了这件事，为他的不忠烦恼不已，几乎要死了。

youdao

God does not want a house built by men, but faithfulness to his word, to his plan.

CNN: Text of Pope Francis' homily

Whereas Biden meets privately with nuns and emphasizes a private faithfulness, Ryan puts his Catholicism right out in the open -- and he attracts some evangelical support for doing it.

CNN: Who will claim the Catholic vote?

In the Bagnolet workshop, Van der Straeten shows me his sketches, and the scientific calculations by which they are rendered with astounding faithfulness to the original thought.

WSJ: Herve Van Der Straeten's Design World | Designer