At first sight, this may be a reference to a common joke about Northern Ireland, about whether a Jew or a Buddhist or an atheist is a Catholic one or a Protestant one.
Bertrand Russell
But back in 1928, Bertrand Russell wrote an essay, "On Catholic and Protestant Skeptics", which he republished in Why I Am Not a Christian. He proposed that their former religions color their attitudes.
He mainly discussed 18th and 19th century ones, and he wrote his essay over 80 years ago, so one must be careful about extrapolating his categories to the present day.
According to him, Protestant freethinkers were acting much like their predecessors, who had left their earlier churches to found new ones. John Stuart Mill recalled his father fervently supporting the Argument from Evil , while admiring the Reformation as a rebellion against priestly tyranny. Virtue is individual and isolated; Bertrand Russell recalled being taught that "thou shalt not follow a multitude to do evil".
However, the change was more wrenching for Catholic freethinkers, who often continued to feel that the Church is a repository of truth and goodness and wisdom, and that they cannot be completely good outside of it. Some of them became rather flippant about systems and deductions and theories and the like, refusing to take them very seriously. However, some others felt the need of a strong church, and some of them have even created new ones. Vladimir Lenin with Communism was the most successful, though strictly speaking he was Eastern Orthodox. Karl Marx, however, was a Jewish/Protestant atheist. Less successful was Auguste Comte with his Positivist Religion of Humanity, which ripped off so much Catholic practice that one critic called it "Catholicism minus Christianity".
For Protestants, this departure was mostly in thought and not in action. A Protestant freethinker might decide that pleasure is what we should live for, but not do much pleasure-seeking, if any at all. However, a Catholic one who decided that might quickly become a pleasure-seeker. BR noted that a Protestant is someone who likes to be good and who has invented theology to make himself good, while a Roman Catholic is someone who likes to be bad and who has invented theology to make his neighbors good.
But BR also noted that some recent Protestant freethinkers would tend to take liberties in action as well as thought, and he called that a result of the general decay of Protestantism (remember that he wrote this back in 1928).
BR did not insist that these were absolute categories; it is possible for Catholic freethinkers to be Protestant-like, and Protestant freethinkers to be Catholic-like. Much more recently, Tim Madigan has written on this issue in The Tower of Babble, noting similar sorts of carryovers.
New Atheism
Most recently, the New Atheists are essentially Protestant-like, even those who are ex-Catholic.