His role of controversial thinker easily led to that of radical social reformer, becoming a provocative gadfly in debates concerning the most pressing issue of his day, slavery. Often intemperate in speech and manner, for fifteen years he embodied militant Christian philanthropy. As a result of his sermons on the subject, many people, including Julia Ward Howe, were converted to the abolitionist cause. Parker's convictions and moral earnestness made him increasingly strident on this issue and kept him in the forefront of radical political activism.