Jacob Acontius (Italian: Jacopo (or Giacomo) Aconcio), c. 1520 – c. 1566, was an Italian jurist, theologian, philosopher and engineer. He is now known for his contribution to the history of religious toleration.
He was one of the Italians, like Peter Martyr and Bernardino Ochino, who repudiated papal doctrine and ultimately found refuge in England. Like them, his revolt against Romanism took a more extreme form than Lutheranism, and after a temporary residence in Switzerland and at Strasbourg (between 1557 and 1558), he arrived in England soon after Elizabeth's accession (1559). He had studied law and theology, but his profession was that of an engineer, and in this capacity he found employment with the English government. 
On his arrival in London he joined the Dutch Reformed Church in Austin Friars, but he was "infected with Anabaptistical and Arian opinions" and was excluded from the sacrament by Edmund Grindal, bishop of London. He was granted naturalization on 8 October 1561. He was for some time occupied with draining Plumstead marshes, for which object various acts of Parliament were passed at this time. In 1564 he was sent to report on the fortifications of Berwick and it appears that he was known in England for both work as an engineer and a religious reformer and advocate of tolerance during the early Reformation.
Before reaching England he had published a treatise on the methods of investigation, De Methodo, hoc est, de recte investigandarum tradendarumque Scientiarum ratione (Basel, 1558, 8vo); and his critical spirit placed him outside all the recognized religious societies of his time. His heterodoxy is revealed in his Stratagematum Satanae libri octo, sometimes abbreviated as Stratagemata Satanae, published in 1565 and translated into various languages. The Stratagems of Satan are the dogmatic creeds which rent the Christian church. Aconcio sought to find the common denominator of the various creeds; this was essential doctrine, the rest was immaterial. To arrive at this common basis, he had to reduce dogma to a low level, and his result was generally repudiated.  Stratagemata Satanae was not translated into English until 1647, but afterwards it became very influential among English liberal theologians.
John Selden applied to Aconcio the remark ubi bene, nil melius; ubi male, nemo pejus ("Where good, none better. Where bad, none worse"). The dedication of such a work to Queen Elizabeth illustrates the tolerance or religious laxity during the early years of her reign. Aconcio later found another patron in Robert Dudley, 1st Earl of Leicester, and died about 1566.
- Stratagematum Satanae libri octo (1565)
- De methodo sive recta investigandarum tradendariumque artium ac scientarum ratione libellus, (1558) (modern edition: De methodo e opuscoli religiosi e filosofici, edited by Giorgio Radetti, Firenze: Vallecchi, 1944)
- Somma brevissima della dottrina cristiana
- Una esortazione al timor di Dio
- Delle osservazioni et avvertimenti che haver si debbono nel legger delle historie
- English translation, Darkness Discovered (Satans Stratagems), London, 1651 (facsimile ed.,1978 Scholars' Facsimiles & Reprints, ISBN 978-0-8201-1313-5).
- Trattato Sulle Fortificazioni, edited by Paola Giacomoni, Giovanni Maria Fara, Renato Giacomelli, and Omar Khalaf (Firenze: L.S. Olschki, 2011). ISBN 978-8-8222-6068-0
- Chambers Biographical Dictionary, ISBN 0-550-18022-2, page 6
- Chisholm 1911.
- Chisholm, 1911: Cal. Slate Papers, Dom. Ser., Addenda, 1547-1566, p. 495.
- Chisholm, 1911: Lords' Journals, vol. i, and Commons' Journals, vol. i., passim.
- Chisholm, 1911: C.S.P. For. Ser. 1564-1565, passim; Acts P.C., 1558-1570, p. 146; his report is now in the Record Office (C.S.P. For. Ser., 1564-1565, No. 512).
- Lynn White, Jr., "Jacopo Aconcio as an Engineer," The American Historical Review 72, no. 2 (1967): 425-44 [doi:10.2307/1859235] and Jean Jacquot, "Acontius and the Progress of Tolerance in England," Bibliothèque d'Humanisme et Renaissance 16, no.2 (1954): 192-206.
- NB for Latin grammar, dropping the two last words justifies the dropping of the genitive.
- Kamen, Henry (1996). "Acontius, Jacobus". In Hans J. Hillerbrand. Oxford Encyclopedia of the Reformation. Oxford University Press. p. 1. ISBN 0195064933.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Aconcio, Giacomo". Encyclopædia Britannica. 1 (11th ed.). Cambridge University Press. p. 151. Endnotes:
- Gough's Index to Parker Soc. Publ.
- Strype's Grindal, pp. 62, 66
- Bayle's Dictionnaire
- G. Tiraboschi, Storia della letteratua italiana (Florence, 1805–1813)
- Österreichisches Biogr. Lexikon
- Nouvelle biogr. générale
- Stephen, Leslie, ed. (1885). "Acontius, Jacobus". Dictionary of National Biography. 1. London: Smith, Elder & Co.