From what I know of this person, he was an Anabaptist/Menonite, who was more known for his anti-Infant Baptism stance, and for his deep appreciation of Mary including her "perpetual" virginity. He also seems to have been somewhat controversial and perhaps even held that it was within Christian discipline to "beat your wife". That idea is an inference from his writings upon the subject of women. He compared God's discipline of his children with a teacher whipping a student, or a man beating his wife.
At any odd's, is he really the most "influential" theologian in Baptist history? How was he even a Baptist? Does not the Baptist tradition trace it's origin to the 17th century via the English Separatists? I think Ergun has some strange ideas about Baptist history and an even worse sense and appreciation of real Baptist's of influence. Imagine Hubmaier being more of an influence than say John Smyth, James P. Boyce, J. L. Dagg, Spurgeon ,Bunyan, Gill and a whole host of others!