A remarkable group within the Barbary corsair cities were the renegades, particularly those of Western European origin. Renegades were originally Christians who had converted — willingly or under compulsion — to Islam.

The success of the Barbary corsairs attracted European adventurers as well, especially experienced captains, mates, and navigators, often men who had previously served English, French, or Dutch privateers. Such men were more than welcome, for they enabled the corsairs to man larger European vessels and thus extend their hunting grounds. There was only one condition: one had to renounce the Christian faith and embrace Islam.

Most appear to have had little difficulty with this requirement. One may question the depth of their devotion to their new religion, yet changing faith and acquiring profits classified as the spoils of war rather than mere theft were tactics that appealed to many pirates.

Under the leadership of these high-ranking renegades, the corsairs succeeded in operating on a far greater scale. With captured European ships at their disposal, they were able to venture far beyond the Mediterranean, extending their sphere of activity into the Atlantic Ocean and even the North Sea. This expansion, however, also created an ever-growing demand for experienced seamen — not only captains and officers, but above all ordinary, seasoned sailors.

Enslaved sailors were therefore generally presented with a choice: convert to Islam and share in the profits of piracy, or spend the remainder of one’s life as a slave and forced labourer. At first glance, that choice may not seem difficult (“better to fight at sea than perish in the quarries”), yet for a European — whether Catholic or Protestant — it posed a profound moral dilemma: by renouncing Christianity, any possibility of returning home was effectively lost forever.

During the course of the seventeenth century, the Dutch authorities were increasingly confronted with fellow countrymen who had converted — voluntarily or otherwise — to Islam. For the community that lost one of its members, this constituted a betrayal of God, Church, and family. Abandoning the Christian faith was regarded as apostasy, condemned with equal severity by Dutch Christians of every denomination. The States General declared that it would be reasonable to punish renegades in the same manner as the Turks punished Muslims who converted to Christianity: by execution.

Source: Muslims in the Dutch Republic, by Benjamin Kaplan (2006).

Some renegades attempted to play both sides against the middle. It was not uncommon for apostates to engage in an elaborate double game, ensuring that they would land on their feet regardless of which side ultimately prevailed. Or, as Miguel de Cervantes writes of a Spanish renegade from Murcia:

“[…] renegades often carry with them, when they intend to return to Christian lands, certain declarations from distinguished captives, in which these men swear, to the best of their ability, that the renegade in question is an honourable fellow who has always treated Christians well, and that he wishes to flee at the first opportunity. Some seek such declarations sincerely; others use them cunningly whenever circumstances require it. If they sail with the Turks on raids against Christian lands and happen to lose their way and fall prisoner, they produce these papers and claim that they prove the purpose of their journey — namely, to remain in Christian territory — and that this is why they had joined the Turks on privateering expeditions. In this manner they escape the first outburst of anger and are reconciled with the Church without suffering punishment; yet as soon as they see their chance, they return to Barbary to become what they once were. There are also those who use these papers honestly, obtain them in good faith, and remain in Christian lands.”

— The Ingenious Gentleman Don Quixote of La Mancha, Part I, Chapter 40.

Among those apostates who preferred to keep a foot in both camps was the Dutchman Jan Janszoon, one of the most dangerous corsairs of his age, who settled in Morocco and adopted the name Murat Reis. Between 1620 and 1640, he led Barbary corsairs as far as Ireland and Iceland in search of slaves. Between his raids, Jan would return to the Low Countries like a respectable Dutch family man to visit his wife and children. Whenever he began to fear for his safety in his homeland, he would once again take ship and resume his violent piratical existence.

In the autumn of his life, when Jan had grown too old to continue raiding abroad yet still longed for his loved ones, he unsuccessfully attempted to persuade his daughter to move to Morocco and keep him company there.

Without these deserters from Christendom, the Barbary corsairs could never have achieved such remarkable success. The renegades brought with them technology, maritime expertise, and navigational knowledge. Although their numbers declined during the second half of the seventeenth century, they continued to play an important role well into the eighteenth century. When a British mission arrived in Morocco in 1727 to ransom slaves, they discovered to their surprise that the sultan’s only cannon foundry was operated not by Moroccans, but by an Irishman.

Source: Historiek, De Barbarijse zeerovers en hun christenslaven

Video on Murat Reis (in Turkish, but subtitled in English).

It is difficult to determine just how large a proportion of the population of Algiers consisted of renegades. Nevertheless, Joost Vermeulen attempts to provide an estimate in his book Sultans, Slaves and Renegades:

"Still, we can make a number of cautious estimates. Several contemporary writers provide us with figures. In 1635, Pierre Dan supposedly counted 9,000 renegades in Algiers, including 1,000 women, nearly all originating from the Hungarian frontier regions of the Ottoman Empire; 4,600 in Tunis (including 700 women); 300 in Salé; and 100 in Tripoli — a total of approximately 15,000. Around 1640, according to Aranda, there were 3,000 French renegades living in Algiers alone, while in 1668 there were said to have been 15,000 renegades in Tunis and Algiers combined. Gramaye claimed that more than 1,500 slaves renounced their faith annually in Algiers, among them 130 Flemings and Dutchmen between 1609 and 1619; the critical Pierre d’Avity, writing in 1637 and in many respects sceptical of Gramaye, nevertheless estimated the annual number of conversions in the Barbary states at around 500.

Naturally, it is extremely difficult — indeed impossible — to derive absolute figures from these estimates, since they are largely unverifiable and seem to be based more upon impressions than upon actual counts. Yet a relative pattern does emerge, especially because the usually reliable Père Dan provides sufficient figures for comparison. He estimated the total population of Algiers at approximately 100,000, which would mean that roughly one in ten inhabitants of the city was a renegade — certainly a conspicuous presence. This ratio appears to hold true even when taking into account the varying population estimates found in different studies and sources. It should immediately be added, however, that the exceptional position occupied by the corsair captains in Algiers and elsewhere may have created the impression that renegades were more numerous than was in fact the case.

How many of these renegades originally came from the Netherlands will probably always remain a mystery. Bennassar examined 1,550 Inquisition trials involving renegades. These individuals had either voluntarily returned from the Christian world or had been captured during raids or military actions. Of 1,523 persons whose nationality could be established, 34 — or 2.23 percent — proved to be Dutch. This percentage may also serve as a rough indication, since the random nature of the surviving trial records is all but negligible, and more than half of the Dutchmen had returned of their own accord. Those who remained in North Africa and left no traces in the archives of the Inquisition naturally do not appear in these proceedings. Moreover, the cases examined spanned the sixteenth and seventeenth centuries, while Dutchmen only began appearing before the ecclesiastical courts from around 1610 onwards. It is therefore clear that their numbers in the seventeenth century were higher than suggested above. Bennassar was also convinced that the Dutch were underrepresented in his sample."