My guess is different from the scholars.  I believe Dietrich Bonhoeffer was truly converted  to Jesus when he was still in New York and was active at Abyssinian Baptist Church.  But here is what the scholars say…

Both the timing of this “great liberation” and its exact nature are exclusive.  Bethge traces the change to 1931-1932 and links it to Bonhoeffer’s dilemma regarding whether a theologian could be a Christian.

Clifford Green locates it in the summer of 1932 and believes it represents the outcome of Bonhoeffer’s struggle with the ego that seeks to dominate others, a battle with personal ambition he had waged since 1927 and that is analyzed in his dissertations.  What we know for certain is that this “great liberation” left a lasting imprint on Bonhoeffer’s thought and behavior and prepared him him for the challenges that lay before him (Stephen R. Haynes and Lori Brandt Hale, Bonhoeffer for Armchair Theologians, 24).