 Ainsworth also wrote reply to John Smyth, who has been called "the first Baptist", entitled "Defence of Holy Scripture, Worship and Ministry used in the Christian Churches separated from Antichrist, against the Challenges, Cavils and Contradictions of Mr Smyth" (1609).Of Smyth's progression to becoming a Baptist, Ainsworth said he 'had gone ‘from error to error, and now at last to the abomination of Anabaptism’, which ‘in him was the worship … of the devil’.His scholarly works include his "Annotations"—on "Genesis" (1616); "Exodus" (1617); "Leviticus" (1618); "Numbers" (1619); "Deuteronomy" (1619); "Psalms" (including a metrical version, 1612); and the "Song of Solomon" (1623).