Dietrich Bonhoeffer wrote letters to his former students of Finkenwalde. But not all of them were circular letters.

Bonhoeffer corresponded with the brethren individually too. He received one letter from one Finkenwaldian who had resisted meditating on the biblical texts. But in the midst of war, he told Bonhoeffer that he kept up the practice on his own. When it was too difficult to meditate on the verses, he simply memorized them, which had a similar effect.

(Bonhoeffer: Pastor, Martyr, Prophet, Spy by Eric Metaxas, 384).