In addition to being an Independent minister, Petto was also a well-respected theologian. His first work was on Pneumatology (doctrine of the Holy Spirit) and it dealt primarily with the doctrine of assurance of salvation and the Spirit's work of sealing. Following mainstream Puritan thought, Petto believed that sealing of the Spirit was a separate act from the indwelling of the Spirit. The sealing of the Spirit granted to individual believers the confidence that God was their Father and that they were truly converted. Later in life Petto seemed to have shifted in his view and like Owen began to see that sealing and indwelling were more closely related, and that both were given to believers upon their initial salvation. He wrote two catechisms,
A Short Scriptural Catechism for Little Children and
A Large Scriptural Catechism, while in Norfolk. The catechism answers were quotations of various biblical passages and the catechisms were designed to encourage memorization of the Bible. Petto also believed that qualified laymen could preach in churches (see above). Petto also seems to have had some ties to the
Fifth Monarchy movement, though it is not clear how closely he was connected. This group wanted the nation to be ruled by Christians and to have the laws based on the
Bible. The name was taken from
Nebuchadnezzar's dream in the book of Daniel, where four kingdoms are represented (
Babylon, Medo-Persia,
Greece, and
Rome). The fifth kingdom was the kingdom of Christ that would possibly be established around the year 1666. He associated with Frederick Woodal and published with John Tillinghast (d. 1655)
Six Several Treatises of John Tillinghast, both leading members of the Fifth Monarchy movement. This close association with this group did not seem to damage Petto's relationship with John Owen, who was one of the leading Independent opponents of the movement. Petto's greatest theological influence came from his highly regarded work on covenant theology,
The difference between the Old and New Covenant Stated and Explained: With an Exposition of the Covenant of Grace in the Principal Concernments of it. In this work Petto skillfully navigated and addressed the many contemporary debates concerning covenant theology showing an exceptional, detailed understanding of the subject. Petto explored the relationship of the Mosaic covenant with the covenant of grace. He made an important connection between the covenants and the Protestant doctrine of justification. His careful work won him the praise of many contemporary theologians including the eminent John Owen, who wrote the book's foreword. ==Influence==