of covenant was extremely important to Puritans, and covenant theology was central to their beliefs. With roots in the writings of Reformed theologians John Calvin and Heinrich Bullinger, covenant theology was further developed by Puritan theologians Dudley Fenner, William Perkins, John Preston, Richard Sibbes, William Ames and, most fully by Ames's Dutch student, Johannes Cocceius. Covenant theology asserts that when God created Adam and Eve he promised them eternal life in return for perfect obedience; this promise was termed the covenant of works. After the fall of man, human nature was corrupted by original sin and unable to fulfill the covenant of works, since each person inevitably violated God's law as expressed in the Ten Commandments. As sinners, every person deserved damnation. Puritans shared with other Calvinists a belief in double predestination, that some people (the elect) were destined by God to receive grace and salvation while others were destined for Hell. No one, however, could merit salvation. According to covenant theology, Christ's sacrifice on the cross made possible the covenant of grace, by which those selected by God could be saved. Puritans believed in unconditional election and irresistible grace—God's grace was given freely without condition to the elect and could not be refused. Conversion Covenant theology made individual salvation deeply personal. It held that God's predestination was not "impersonal and mechanical" but was a "covenant of grace" that one entered into by faith. Therefore, being a Christian could never be reduced to simple "intellectual acknowledgment" of the truth of Christianity. Puritans agreed "that the effectual call of each elect saint of God would always come as an individuated personal encounter with God's promises". The process by which the elect are brought from spiritual death to spiritual life (regeneration) was described as conversion. Early on, Puritans did not consider a specific conversion experience normative or necessary, but many gained assurance of salvation from such experiences. Over time, however, Puritan theologians developed a framework for authentic religious experience based on their own experiences as well as those of their parishioners. Eventually, Puritans came to regard a specific conversion experience as an essential mark of one's election. The Puritan conversion experience was commonly described as occurring in discrete phases. It began with a preparatory phase designed to produce contrition for sin through introspection, Bible study and listening to preaching. This was followed by humiliation, when the sinner realized that he or she