Written by: Ted Vial
Scripture for Reformed/Presbyterian churches means first and foremost the Bible. While this is true for most kinds of Christianity, several factors make it particularly crucial for Reformed churches. First, Reformed theologians were influenced by the medieval nominalists and their belief that, in our fallen state, God was completely unknowable, except insofar as God chose to reveal certain things to humans. The revealed word of God is contained in scripture; hence the principle is sola scriptura-scripture alone. Second, there was no human work that could save you, or influence God to save you. Your only hope for salvation was the free gift of God's grace, sola gratia. The promise of this grace is given in scripture. A third key Protestant principle is justification by faith alone, sola fide. Here, too, God completes the work of salvation without any human effort; the individual receives through faith what God has done.
These principles led Zwingli and Calvin to grant scripture final authority over all others, including the Church and human reason. Calvin argued that there are no rational arguments to persuade people that the Bible is the source of truth. The same Spirit present in the Bible is present in the hearts of those who turn to the Bible as truth; in Calvin's phrase, scripture is "self authenticating." The Reformed churches date, in a sense, to Zwingli's decision to preach straight through the Gospel when he arrived in Zurich, confident that the text itself was the basis of a pure Christian life.
The Reformed Bible is not quite the same as the Catholic Bible. There were several books that Zwingli and Calvin (as well as Luther) felt did not have canonical authority and had been added in error by the Catholic Church. These are mostly late texts (considered by Protestants to be "inter-testamental"). They used a list of Hebrew texts that ancient rabbis considered canonical. Books considered canonical by Catholic but not Lutherans and other Protestants are gathered in the Apocrypha.
There are several other writings, including creeds and catechisms, which do not have the same status as scripture for Reformed and Presbyterian churches, but which they refer to officially to distinguish their understanding of the Bible and of Christianity from other Christian churches, including other Protestant churches.
First and foremost is Calvin's Institutes of the Christian Religion. By the final edition it had expanded from a pocket-sized book to a multi-volume work. This is one of the great works in systematic theology in Christian history. (A systematic theology takes up all the important doctrines in turn, from creation and the nature of God, through human sin and salvation, to eschatology or the end of history.) Other reformers wrote occasional theology arguing for or against specific beliefs in specific circumstances. Calvin, a second-generation reformer, took up the main Protestant beliefs and carefully laid them out and showed how they fit together.