The Correspondence of Guglielmo Sirleto

Primary Contributors:

Jan Machielsen

Portrait of Guglielmo Sirleto. Seventeenth-century. Engraving. (Image reproduced by permission of Jan Machielsen.)

Guglielmo Sirleto (1514–1585)

Guglielmo Sirleto was the ‘Forrest Gump’ of the Counter-Reformation, both because he was almost always in the rooms where the action happened, and because—as Irena Backus and Benoît Gain noted in their important essay—so few historians have taken note of him. As an assistant to Marcello Cervini (briefly Pope Marcellus II), he was an important observer of the Council of Trent. He was the teacher of Carlo Borromeo and close to Philip Neri, who, hagiography has it, sold his books to support Sirleto as a poor student. He served on many, if not all, of the papal commissions which emerged out of the Council’s deliberations on the reform of the calendar and the liturgy, and on the revision of the Bible. As prefect of the Vatican Library, and later cardinal librarian, Sirleto had access to—and acted as gatekeeper for—Europe’s most valuable collection of manuscripts, often identified by early modern Catholics with tradition itself.

Sirleto personifies how the different impulses of early modern Catholic Reform—the repressive institutions played up by historians past, and the missionary efforts emphasized by historians present—really are only two sides of the same coin. Not only did he oversee and facilitate the production (and translation) of new corrected patristic and liturgical texts, he was also Prefect of the Congregation of the Index. Much the same argument can be made for Sirleto’s interest in and knowledge of Hebrew and ancient Greek, which were put to practical use. His interest in fostering relations with Eastern Christians was easily compatible with his critical attitude towards those still using the Greek rite in Italy. His role as cardinal protector of neophytes, similarly, involved both carrot and stick, supporting converts from Judaism while making the lives of their erstwhile co-religionists difficult. (He heartily approved of the burning of copies of the Talmud on the Campo de’ Fiori in 1553.)

Sirleto’s interest in these different types of conversion shows once more—if this is still even worth noting—how outmoded the view of early modern Catholicism as a reaction to the Reformation is. In fact, Protestantism constituted only a marginal interest in his correspondence. And while this partly reflects the nature of Sirleto’s correspondence network, which only rarely stretched beyond Italy and its environs (Malta, Croatia, Savoy, Venetian outposts in Greece), it also shows just how much a Catholic arm-chair Reformer had to get on with.

Partners and Additional Contributors

This project was made possible by a British Academy Small Grant and by the hospitality of the British School at Rome. Jan Machielsen is grateful to Simon Ditchfield for commenting on the draft application and for acting as his referee, and to Madeline McMahon for her comments on a draft version of the text below. . He would like also to thank the hospitality of the staff of the Biblioteca Apostolica Vaticana [BAV], in particular András Németh.

Key Bibliographic Source(s)

Some of the letters in this—partial—catalogue were also included in the BAV’s online catalogue. A number of the manuscripts are also available online on the BAV website, digitized from ‘low-quality’ (the library’s own description) microfilm. Please see the discussion of the contents of this catalogue and its limitations below.


This catalogue is one of three emerging out of a British Academy Small Grant project entitled ‘Making a Church Ever the Same: Catholicism between Rome and the Borderlands, c. 1550–1620’. It comes, as all catalogues should, with a health warning in the form of an account of its origins. I had looked at Guglielmo Sirleto’s correspondence during the final stages of writing my first monograph on the Flemish-Spanish Jesuit Martin Delrio. I was struck, then, by the content and tone of letters by Netherlandish scholars such as Laevinus Torrentius, Jacobus Pamelius, and especially Willem Lindanus. Not only did they turn to Sirleto for help, they also regarded the Vatican Library as an arsenal to be employed against the heretics.

The British Academy Small Grant enabled a two-and-a-half-month stay to go through Sirleto’s correspondence systematically, and those of other prominent scholars, essentially in search for correspondence from beyond the Italian peninsula. As the British Academy Small Grant scheme requires a concrete output, the idea emerged to create an inventory as an intermediate output, a stopping place if you will, on the road to the eventual book manuscript. As already indicated above, most of Sirleto’s correspondence did not fall in the category of (transalpine) scholarship, although many letters turned out to be extremely interesting in their own right. I shared via e-mail those that I believed were of potential interest to academic colleagues and posted abstracts of some of the most interesting on Twitter using the hashtag #PopishPost. (These include a letter from the Patriarch of Venice reporting that Guglielmo Sirleto Neofito, a convert bearing the cardinal’s name, had absconded from Venice for Constantinople with stolen money; and one arising from a diocesan visitation where a priest was discovered to have used the wrong formula for baptism.)

The caveat, then, is that this catalogue is offered with the hope that it may be useful to others, but that it was never an end in and of itself. If it had been, further resources would have been needed, other practices employed, and also more people—e.g. with greater expertise in Italian history and geography—engaged, to ensure better accuracy. This catalogue should be seen foremost as an exercise in open note-taking, and as such it comes with an open invitation to send in corrections and additions.

Although there are occasional Sirleto letters scattered across the collections of the Vatican Library, the bulk of the correspondence is grouped closely together. It, in effect, consists of four parts:

1.–79: these volumes relate to the Council of Trent, containing letters from Sirleto to Marcello Cervini (1547–1553;, letters from Cervini to Sirleto (same period;, and letters from Sirleto to Girolamo Seripando (1562–63).

2.–86: these volumes contain letters addressed to Sirleto, almost exclusively dated to the last two decades of his life, following his elevation to the cardinalate in 1565. These letters are arranged thematically: princes (; with some stray items at the end); cardinals (; bishops (; nobles (; other notables, magistrates, and so on, (, clergy (mostly;, and family ( Several of these volumes consist of two parts with the folios numbered consecutively. Within these volumes, few clear organizational principles can be discerned. They generally consist of roughly chronological bundles bound together, but these bundles themselves do not follow each other chronologically.

3.–96: these volumes also contain letters addressed to Sirleto. They are arranged chronologically, though with errors especially where Roman dates are concerned: up to 1568 (, mostly dating to the 1560s), 1569 and 1570 (, from 1571 to 1573 (, from 1574 to 1577 (, from 1578 to 1580 (, 1581 and 1582 (, and from 1583 to 1585 (

4. a manuscript of 426 folios, containing copies of letters sent by Sirleto during his cardinalate. The last letter to the Archbishop of Milan is dated 25 September 1585, two weeks before the cardinal’s death. These letters are copies of letters sent, not drafts (as indicated by postscripts written ‘di man[o] del Card[ina]le’). This volume is unlikely to contain all of Sirleto’s outgoing correspondence, however. His period as bishop of San Marco Argentano is explicitly missing.

These manuscripts entered the collections of the Vatican Library upon Sirleto’s death. His Will explicitly left the first set of letters to the Vatican Library, as well as (among some other writings) ‘all other writings and documents pertaining ad usum Sanctae Sedis Apostolicae’, which may well cover the rest. (The remainder of his library was put up for sale and an attempt was made to interest Philip II of Spain.) The precise organization of Sirleto’s correspondence at this point is unclear. A survey (BAV, Arch.Bibl.11, fols 157–158) drawn up by Federico Ranaldi, Sirleto’s faithful secretary and the Library’s custodian, of the documents received from the cardinal’s heirs list specific sets of letters that can be matched to items still in the Vatican collection (for example, compare ‘lettere al Card[inale] Sirleto dal Arciv[escov]o di Salerno come è estate trovato il corpo di Papa Gregorio [i.e. VII]’ with, part 1, fol. 169). This may suggest that the present (and awkward) organization of Sirleto’s correspondence was put in place after his death. Certainly, it was already established by 1643 when the letters appear with their present shelf marks in a catalogue drawn up by another member of the Ranaldi family. (See Sala.cons.mss.307.rosso.) A complete catalogue of the Sirleto correspondence would help unravel the mystery of its present organization.

As this survey also suggests, pursuing Sirleto’s exchange with a particular correspondent requires the consultation of manuscripts from across these series (that is, the letters of for instance, bishops, can also be found in the chronological series). Some famous items have been published separately, for instance, the single letters from Cesare Baronio (seeking permission to read the Magdeburg Centuries) and Roberto Bellarmino (on the Septuagint). Similarly, exchanges with some correspondents (Seripando, Borromeo, Granvelle) have been published, often in fairly out-of-the-way places.

The content of many of the volumes has been listed in the Vatican Library’s online catalogue, drawn from the early twentieth-century card catalogue. This catalogue omits some volumes entirely ( and In other instances, it includes only a selection. The extent of this lacuna is obscured by the fact that multiple letters (and thus multiple folio numbers) from the same correspondent are listed as a single entry (and originally on a single card). Gaps are consequently less easy to identify. Where more than two (or three) items are listed, places and dates of the intervening letters are usually omitted—likely because space was lacking on the original card file. Although all these volumes were microfilmed and are available in digital format within the library itself, only a (seemingly arbitrary) selection is presently available online. (Their quality, as already noted, is nothing to write home about.)

There is thus a compelling case for developing a new catalogue, which integrates the different parts of Sirleto’s correspondence. The complexity of this task was not foreseen when this project began, and as already stated, the development of such a catalogue was never its end goal. The following catalogue thus contains only a subset of the letters:,, (both parts), (part 2 only), (both parts), and (both parts). A conservative estimate suggests that these 1,438 letters make up about a quarter of the total correspondence. At present, there are no immediate plans (or funding) to take this further. One short-term aspiration is to gather those signatures (from letters already available online) that could not be fully deciphered for feedback from the wider community. The value of Sirleto’s correspondence is manifold, but it certainly lies in the fact that he received letters not only from the great and the good, but also from ordinary folk, such as lowly parish priests and recent converts, who have received no entries in Treccani, EDIT16,, or CERL, and whose signatures were not always legible. Corrections and, indeed, additions are welcome.

Jan Machielsen

Further resources

For further reading on Sirleto, see:

Benedetto Clausi and Santo Lucà, eds, ‘Il ‘Sapientissimo calabro’: Guglielmo Sirleto nel V centenario della nascita (1514–2014); Problemi, ricerche, prospettive’, Quaderni di Νἐα Ῥώμη, 5 (Rome: Università degli Studi Roma ‘Tor Vergata’, 2018).

Peter A. Mazur, Conversion to Catholicism in Early Modern Italy (London: Routledge, 2016), in particular chapter 1.

Irena Backus and Benoît Gain, ‘Le Cardinal Guglielmo Sirleto (1514–1585), sa bibliothèque et ses traductions de saint Basile’, Mélanges de l’école française de Rome, vol. 98/2 (1986), pp. 889–955.

Georg Denzler, Kardinal Guglielmo Sirleto (1514–1585): Leben und Werk; Ein Beitrag zur nachtridentinischen Reform (Munich: Max Hueber Verlag, 1964).

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