 On reviewing the history of the Church with respect slavery Maxwell (1975) concludes that "In Catholic countries the abolition of slavery has been due mainly to humanist influences".The political philosopher Luigi Sturzo argued that the change in attitude to slavery among many Christian thinkers followed its legal abolition rather than preceding it.In 1839, Pope Gregory XVI issued a bull, with the incipit "In supremo apostolatus" in which he condemned slavery, with particular reference to New World colonial slavery and the slave trade, calling it "inhumanum illud commercium."