Ad fontes (Latin "to the sources") A watchword of Renaissance Humanist scholars. They urged a return to the original sources of Greek and Roman classics, the Scriptures, and the writings of early Christian theologians. This approach influenced Protestant Reformers such as John Calvin (1509-1564).
The following is from D. A. Carson and Douglas J. Moo, An Introduction to the New Testament, Second Edition (Grand Rapids, MI: Zondervan, 2005), 44-46.
The Renaissance, a period of European history that historians customarily attach to the fourteenth, fifteenth, and sixteenth centuries, witnessed a “rebirth” (which is what 'renaissance' means) of classical culture. The printing press was invented, the influence of which cannot easily be overstated. Constantinople fell to the Muslim Turks in 1453, which sent not a few scholars scurrying to the West, bringing their Greek manuscripts with them. The rise of learning and the founding of several European universities trumpeted the call, Ad fontes—“to the sources.” The study of Greek and Hebrew became commonplace; the authority of Latin was increasingly displaced. The renewal of interest in both Christian and pagan foundational documents produced a growing number of informed and highly literate “humanists” who were more than willing to criticize the clerical abuse then rampant at almost every level of the Catholic Church. By and large, the humanists in northern Europe became more interested in the classical Christian texts (the New Testament and the patristics) than in the classical pagan texts, and they have thus sometimes been labeled “Christian humanists.” The most influential of these was Erasmus of Rotterdam, whom we have already met.
Those influenced by the Renaissance also became increasingly suspicious of the four interpretive levels that had been justified by the theologians of the Middle Ages. They wanted to read the primary sources for themselves, and they tried to read them more “literally” or more “naturally.”
Scholars still dispute the nature of the relationships between the Renaissance and the Reformation (sixteenth century). Certainly the demand for reform increasingly voiced by Christian humanists contributed to the growing unrest in Western Christendom. That fact generated the old saw that “Erasmus laid the egg that Luther hatched.” Moreover, many younger humanists converted to Protestantism, including such leaders as Ulrich Zwingli (d. 1531), Philipp Melanchthon (d. 1560), John Calvin (d. 1564), and Theodore Beza (d. 1605).
The Reformation emphasis on sola scriptura (“Scripture alone”) embraced in practice several emphases. Over against the Catholic view that revelation is a deposit entrusted to the church, a deposit of which Scripture is only a part, the Reformers insisted that while there is much to learn from Christian tradition, much indeed that holds us to account, only the Bible has final authority. The Bible must not be domesticated by the tradition. This emphasis had two complementary effects: (1) Ideally, the Scriptures should be studied in the languages in which they were written; and (2) the Scriptures should be disseminated as widely as possible, which meant that vernacular translations should be prepared. The aim of the Bible translator William Tyndale (strangled and burned in 1536) was to make the ploughboy as knowledgeable in the Bible as the high prelates of the church. Moreover, insistence on “Scripture alone” prompted the Reformers to study once again what constitutes Scripture, and this led to the rejection of the Apocrypha as part of the canon. The fact that the Catholic Church adjudged these books (the exact number of them is somewhat disputed) to be canonical or “deuterocanonical”—that is, canonical in a secondary sense—was not a sufficient reason for hanging onto them. Indeed, at one stage in his life Martin Luther questioned the authority of the canonical James (“a right strawy epistle,” in his famous phrase).
Partly under the influence of Renaissance learning, the Reformers learned to be suspicious of the fourfold hermeneutic they had inherited. This does not mean they became crass literalists. They could recognize (as all good readers can) metaphors and other figures of speech. They wrestled with what would today be called typology. The fact that the Bible is often talking of eternal things in the categories of everyday temporal things prompted Luther to think of Scripture as a litera spiritualis. One may doubt that this is the most helpful analysis, yet it is vital to recognize that although the Reformers dismissed as artificial the fourfold interpretive approach defended in the Middle Ages, they were not unaware that the “natural” reading was not always straightforward. Moreover, the efforts of both Luther and Calvin (to go no farther) to write both commentaries on books of the Bible and expositions of Christian doctrine had the effect of tying doctrine to the Bible itself. Indeed, Calvin’s enormously influential Institutes of the Christian Religion was meant to be a kind of accurate introduction to what the Bible teaches. This work wrestles endlessly with Scripture yet works out its doctrinal formulations in interaction not only with issues of importance when Calvin was writing but also in interaction with eminent Christian thinkers throughout history. In conjunction with Calvin’s commentaries, the Institutes taught many generations of believers what to believe and how to think. Inevitably, works such as these constituted models for the interpretation and the teaching of Scripture. It became impossible to try to understand the New Testament, let alone the entire Bible, without reflecting on such work.