The Bible, consisting of 66 "books" (or major units), is scripture for Protestant churches. The canon of the Protestant Bible is not quite the same as the Roman Catholic Bible. There were several books that Roman Catholics included in the Old Testament that Luther, Zwingli, and Calvin felt did not have canonical authority, and had been added in error by the Catholic Church. These are mostly late Jewish texts (considered by Protestants to be "inter-testamental"). The reformers used instead a list of Hebrew texts that ancient rabbis considered canonical. Books considered canonical by Catholics but not Protestants are gathered in the Apocrypha.
While the centrality of the Bible is crucial for most Christians everywhere, several factors make it particularly essential for Protestants. First, Protestant 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. (One Protestant principle is sola scriptura—scripture alone as the supreme authority for faith and practice.)
Second, there was no human work that could save you, or move God to save you. Your only hope for salvation was the free gift of God's grace, a principle called sola gratia. The promise of this grace is given in scripture. This grace is accessed only through faith, thus the Protestant principle of justification by faith alone, called sola fide. These principles–by scripture alone, by grace alone, by faith alone—led Luther, Zwingli, and Calvin to grant scripture final authority over other sources of authority, including the Church and human reason. They did not discount these other voices, but simply elevated that of scripture, so that scripture became the final arbiter of truth. Thus Church tradition could be criticized on the basis of its agreement with the higher authority of the scripture. The Roman church had argued that the tradition of the Church, as expressed and administered by the magisterium, embodied scriptural truth, and thus any perceived discrepancy in theory or practice between what the Bible said and what the Church said could be explained by the Church's interpretation.
Luther argued that correct interpretation of scripture rests not with the Church but "in the heart of the pious believer," who is, of course, a member of the Church. Despite later, more radical, applications of this argument, Luther taught that scriptural interpretation was not a matter for isolated individuals, but for individuals within the Christian community who could draw on the wisdom of the tradition, study the texts, and come to common conclusions. The contrast was between the individual's direct access and encounter with scripture and the Church's dictates regarding how and what to believe.
Essentially agreeing with Luther, Calvin went on to argue 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.")
Some Protestant churches have produced creeds, or formal statements of Christian belief, that function as a guide to correct interpretation of scripture for that denomination. Some denominations declare themselves to be "non-creedal," that is, they reject the creation of formal statements of faith and claim instead to rely on direct encounter with the Bible. Most of these stem in some way from the Anabaptists. Non-creedal churches include Baptists, Churches of Christ, and Mennonites. Many "non-denominational" churches, often growing out of the Pentecostal movement, are also non-creedal. Among these non-creedal traditions, most have found it necessary, however, to formulate what they call "confessions" or more recently, "statements of faith." These traditions make a distinction between "creeds," which they view as being incapable of being reformed or changed by the teaching of scripture, and "confessions," which, at least in principle for them, are capable of being revised if found to be inconsistent with scripture and, therefore, do not threaten the supreme authority of the Bible.
Lutheran Churches rely on several creeds gathered in the Book of Concord to guide and set limits around correct interpretation of scripture. There is no single, agreed-upon book of creeds or confessions for Reformed Christians, but the more widely recognized ones include The Westminster Confession, as well as The Scots Confession, the Heidelberg Catechism, and the Second Helvetic Confession. They also include important 20th-century statements: the Theological Declaration of Barmen (written largely by Karl Barth against Nazism), and the Confession of 1967 (written as part of an effort to unify separate Reformed churches).