This quote is from the Preface to Narrative of Sojourner Truth (http://bit.ly/2ljP3bX), which was dictated to Olive Gilbert, who edited the work and presumably wrote the Preface. Here is the context:

“A Chicago lady wrote to her, asking for a thought to inspire and cheer her on her life journey. Sojourner responded as follows : — ‘God is from everlasting to everlasting.’ ‘There was no beginning till sin came.’ ‘All that had a beginning will have an end.’ ‘Truth burns up error.’ ‘God is the great house that will hold all his children.’ ‘We dwell in him as the fishes in the sea.’ Of the fashionable so-called religious world she says, ‘It is empty as the barren fig-tree, possessing nothing but leaves.'”