Another milestone in the early development of Baptist doctrine was in 1638 with John Spilsbury, a Calvinistic minister who helped to promote the strict practice of believer's baptism by immersion. According to Tom Nettles, professor of historical theology at Southern Baptist Theological Seminary, "Spilsbury's cogent arguments for a gathered, disciplined congregation of believers baptized by immersion as constituting the New Testament church gave expression to and built on insights that had emerged within separatism, advanced in the life of John Smyth and the suffering congregation of Thomas Helwys, and matured in Particular Baptists."