Libertine, from the Latin word meaning freemen, was the name given by John Calvin to his opponents in Geneva, mostly from rich and established families, who believed that the city should not be under religious control and that non-Calvinists should not be subject to Calvinist religious law, as well as claiming that the Consistory of Geneva, which was the religious governing body in Calvinist Geneva, had no right to excommunicate Genevans. Calvin's struggles with the Libertines made up most of the conflict he faced in Geneva, as they controlled much of Geneva's government, and it was not until the mass migration of French Calvinist refugees to Geneva that control of the government slipped out of their control.
Because of Calvin's opposition to the Libertines, the term has been made into a term of abuse.
In addition to Calvin's opposition, the Marquis de Sade was the other major influence in redefining the meaning of the word, as he self-identified as one. Currently, A Libertine is someone with no morality, completely selfish with no empathy, and who pursues sexual pleasure in every way possible. They are almost entirely male. Their lack of morality naturally makes them an atheist, and some even take pleasure in blasphemy (presumably to convince themselves that no damnation awaits them). At the very minimum, for one to be a libertine, they must be a bisexual, non-exclusive pedophile. They are also highly likely to have other paraphilias as defined in the DSM-IV, such as sadism, masochism, voyeurism, and/or exhibitionism. Essentially, libertines are sexual deviants taken to the furthest extreme- if anyone self-identifies as such, call the police because they have almost certainly committed heinous sexual and/or violent crimes.