Zeal of the convert

The zeal of the convert is a phrase describing the typically fierce devotion of someone whose belief system has changed because of personal experience or argument, especially as compared to someone who has held their beliefs since childhood. In addition to some empirical support of the phenomenon, the occasionally excessive fervency of converts has long been proverbial.

While typically spoken of in reference to religious beliefs, it is not uncommon to hear this phrase used to describe those with newly-held political ideology.

Examples

 * Paul of Tarsus, a Pharisee Jew who persecuted Christians, would go on to be one of the most devout and pivotal figures among the early Christians. However, given that he is the only source for his supposed anti-Christian zealotry prior to converting, he may well have exaggerated it to make a point which is surprisingly common in Christian apologetics.
 * Martin Luther, leading figure and theologian of the Protestant Reformation in Germany, and former Roman Catholic monk who denounced corruption and opulence in the Catholic Church, and, among other things, violence towards Jews to be against God's plan; he was initially somewhat sympathetic to Jews but ended up inspiring the Nazis' policies with his pamphlet On the Jews and Their Lies.
 * Ultraconservative American radio host Michael Savage was once a liberal consort of hippies, only to later become one of the angriest of all the media's opponents of liberalism.
 * Salem Radio hosts Michael Medved and Dennis Prager both claim to be former liberals prior to taking a hard-right, anti-liberal turn.
 * Professor Ronald Radosh went from being a radical Marxist (arguably pro-Maoist even) to an ardent neoconservative. Similarly former Trotskyist leader James Burnham eventually became regarded as the original neoconservative, and many former Communists such as David Horowitz have taken similar shifts in their positions (from far-left to arch-conservative).
 * Many former British socialists such as Melanie Phillips and Peter Hitchens would later become nationalistic wingnuts.
 * Benito Mussolini going from a Democratic Socialist to an anti-socialist fascist.
 * Thomas Sowell going from being a Marxist sympathizer to a very right-wing libertarian.
 * Maajid Nawaz going from being a recruiter for extreme Islamist group Hizb ut-Tahrir to speech writer for David Cameron, collaborator with Sam Harris and becoming a moderate Muslim, only for him to become a conspiracy theorist in recent years and sympathised with the 2021 U.S. Capitol riot insurrectionists.
 * Manny Pacquiao, a Filipino former boxer turned politician. Used to carry a rosary and make the sign of the cross before and after each fight, prior to being swayed into Evangelical Christianity and base his sociopolitical views on right-wing theocracy.
 * Neoconservatism originated in the US among leftist, Trotskyite, Marxist and Maoist circles when they argued that the US should use its military might "for good", i.e. to "spread democracy"

The political conversions in recent years, especially from "liberal to wingnut", are a little dubious at times, with some cynical liberals and moderate conservatives (such as Mike Lofgren) pointing out that the concept of "redemption" has become a big feature among the religious right, so claiming to be a liberal who has "opened their eyes" and seen the "truth" can be a good way to get sympathy from their target audience. That being said the "conversion" is often genuine, and in recent years has tended towards a hard-lefty going to the hard-right, perhaps because the fall of Communism and rise of reactionary politics in the western world gave a more appealing "black-and-white" political philosophy tending toward the political right.

Dubious examples, or the repentant sinner
A variant of the zealous convert is what may be called the repentant sinner, also known as argumentum ad Schnoebelenum or argumentum ad Warnkeum. Here someone is claiming to have been completely opposed to the views they have now converted to, which is of course a good way to make the conversion story seem all the greater. The problem is that the depiction of the convert's prior life of sin is either unsubstantiated or can be conclusively shown to be wrong and is in any case simply implausible. Paul of Tarsus may have been an early example (we only have his word for his prior anti-Christian fervour after all), but more recent cases include:
 * Lee Strobel, who claims to have been a staunch atheist before becoming a born again Christian — the problem is that his depiction of his supposed atheism is not only completely undocumented until after he launched his career as an apologist, but Strobel's depiction of his atheism in The Case for Christ sounds an awful lot like the fundamentalist Christian atheist straw man (e.g. that Strobel actually knew that God existed, but simply denied Him to be able to sin freely).
 * Mike Warnke and several other fabricators who helped start the Satanic ritual abuse phase of the 1980s' Satanic Panic. They all made up sensational bullshit stories about their past in Satanic cults and each and every story was shown to be demonstrably false.
 * Kirk Cameron claims to have been an atheist while younger. The fact that his conversion to far-right evangelical Christianity happened at the age of 17 makes one suspect how deeply entrenched his atheism actually was.
 * C.S. Lewis who, similar to Lee Strobel, claimed to be an atheist convert to Christianity but whose description of atheism seemed to include the idea that he always knew God existed, and only denied it. In his autobiography Surprised by Joy, Lewis states:


 * "You must picture me alone in that room in Magdalen, night after night, feeling, whenever my mind lifted even for a second from my work, the steady, unrelenting approach of Him whom I so earnestly desired not to meet. That which I greatly feared had at last come upon me. In the Trinity Term of 1929 I gave in, and admitted that God was God, and knelt and prayed: perhaps, that night, the most dejected and reluctant convert in all England."


 * Although, unlike Strobel, Lewis seems to have genuinely rejected Christianity at some point, as opposed to inventing a false backstory.


 * Too many to count people, not few of them Fundamentalists, who claim to have been a former atheist, homosexual, etc. until they discovered Jesus and that it led them to a righteous life. However you dig on their claims and you find things are more or less sketchy.