Galileo Galilei's "Letter to Benedetto Castelli" (1613) was his first statement on the authority of scripture and the Catholic Church in matters of scientific enquiry. In a series of bold and innovative arguments, he undermined the claims for Biblical authority which the opponents of Copernicus used. The letter was the subject of the first complaint about Galileo to the Inquisition in 1615.

In 1610 Galileo had published Sidereus Nuncius (The Starry Messenger), which made him famous across Europe. This work prompted many debates as to whether the Earth really was the centre of the universe. Galileo usually avoided referring to scripture in his arguments about the universe, while the Aristotelian scholars who opposed Copernicus cited the Bible in support of their views – for example Lodovico delle Colombe in his 1611 work Contra il Moto della Terra (Against the Motion of the Earth) explicitly challenged anyone defending Copernicus to answer the charge that he was going against what the Bible taught.

This presented Galileo with a dilemma – if he did not respond, he effectively conceded that the biblical text confirmed the Aristotelian view despite the fact that the Church had no firm position on the Copernican question; on the other hand if he tried to engage in arguments based on scripture, he allowed himself to be drawn into a field where the Church regarded its authority as absolute. Indeed an earlier venture into this form of argument by Galileo had been stopped by the Church. In the manuscript for Letters on Sunspots (1613), he argued that "flaws" in the Sun demonstrated that the heavens were not immutable, as had previously been thought. A paragraph in which Galileo supported this by claiming that the scriptures supported the mutability of the heavens was removed by the Inquisition censors.

On 14 December 1613 Galileo's friend and former pupil Benedetto Castelli wrote to him to say that at a recent dinner in Pisa with the Grand Duke Cosimo II de' Medici a conversation had taken place in which Cosimo Boscaglia, a professor of philosophy, argued that the motion of the Earth could not be true, as it was contrary to the Bible. Castelli had disagreed with him and maintained, as Galileo held, that the Earth's motion was possible. After the dinner, Castelli had been called back by the Dowager Duchess Christina of Tuscany to answer points she raised from scriptural arguments against the motion of the Earth. Castelli had responded and Boscaglia had remained silent. Castelli wished to alert Galileo to this exchange, and advised Galileo that their mutual friend Niccolò Arrighetti would come to Florence and explain matters further. This Arrighetti did.

Galileo felt that it was important for him to set out an argument to show how scripture could not be used as the basis for scientific enquiry. He did so with great speed, replying with a letter to Castelli in less than a week, on 21 December 1613. His Letter to Benedetto Castelli was not published, but was circulated widely in manuscript form. As the debate about its arguments continued, Galileo thought it advisable to review and expand the arguments he had set out. This was the basis of his subsequent Letter to the Grand Duchess Christina, which expanded the eight pages of his letter to Castelli to forty pages.

In his letter to Benedetto Castelli, Galileo argues that using the Bible as evidence against the Copernican system involves three key errors. Firstly, claiming that the Bible shows the Earth to be static and concluding that the Earth therefore does not move is arguing from a false premise; whether the Earth moves or not is a thing which must be demonstrated (or not) through scientific enquiry. Secondly, the Bible is not even a source of authority on this kind of question, but only on matters of faith - thus if the Bible happens to say something about a natural phenomenon, this is not sufficient for us to say that it is so. Thirdly, he shows by deft argument that it is open to question whether the Bible, as his opponents claimed, even contradicted Copernicus' model of the universe. Indeed, Galileo argues, a key passage in the Bible which was held by his opponents to support the view that the Sun moves round the Earth supports his own views much better.

In the Bible Joshua 10:12 is an account of how God commanded the Sun to stand still so that Joshua could defeat his enemies. According to those opposed to Copernicus, this showed clearly that the Sun (and not the Earth) moved. Galileo argued that this passage could not be used to support the traditional Earth-centred view of the universe at all. If we assume the universe to be as it was described by Claudius Ptolemy, the Sun's annual motion was a slow movement towards the East, so if God had commanded it to stop, the daily movement towards the West would no longer have been counteracted and as a result the day would actually have got slightly shorter rather than longer. However if we assume the universe to be as Copernicus described it, the Sun is at the centre and its rotation drives the rotation of all the planets. Thus if God had ordered the Sun to stop turning, everything would have stopped and the day would have been longer, just as the Bible described.

Two aspects of Galileo's letter are particularly worthy of note. First, his boldness in venturing into the field of exegesis, where he was bound to upset many theologians who would not welcome his contributions. Second, his rhetorically brilliant argument was based on a fundamental contradiction; he began by arguing that faith and science were distinct, and that the Bible could not be used as the basis for arguments about science; he then went on to show, according to some novel and clever arguments, that actually the Bible supported his own scientific views.