When Dietrich Bonhoeffer was in Barcelona in 1928, he served as a pastor in a German community. Eric Metaxas wrote the following on the sermons of Bonhoeffer…

Bonhoeffer’s sermons challenged the congregations both spiritually and intellectually. In his first sermon he leaped into his favorite subject, the difference between a faith based on our own moral efforts and one based on God’s grace. Along the line he mentioned Plato, Hegel, and Kant, and quoted Augustine. One can only imagine some of the Barcelona businessman puzzling over this earnest twenty-two-year-old, freshly descended from the ivory tower.

And yet there was an undeniable vitality to what he was saying; he rarely lost their attention.    

(Bonhoeffer, Pastor, Martyr, Prophet, Spy by Eric Metaxas, 77).