Pope Francis has spoken in unusually frank terms about the theological divide in the U.S. Catholic Church, calling it an “honour” that some conservative Catholic groups in the country continue to criticize his papacy.

In a brief exchange with a journalist aboard his Sept. 4 flight from Rome for the beginning of a three-nation tour of Southern Africa, the Pope was presented with a new book that details years of efforts by conservative U.S. Catholics to influence his decision-making.

Francis told Senèze that he had heard about the book, published in France and titled How America Wanted to Change the Pope, but had yet to read it. Passing the volume to an assistant, the pontiff joked: “It’s a bomb!”

Criticism from conservative U.S. Catholics has been a staple of Francis’ six-year papacy, with right-wing outlets such as EWTN and First Things taking aim at many of his initiatives, such as his effort to fight global climate change and his focus on the merciful nature of God.

Shortly afterward, Vatican spokesman Matteo Bruni sought to downplay the importance of Francis’ remarks, saying the Pope was speaking in an “informal context” and “wanted to say that he always considers criticism an honor, especially when it comes from authoritative thinkers, in this case from an important nation.”Francis’ Sept. 4-10 voyage is only his second to sub-Saharan Africa. He will first visit Mozambique, and then the island nations of Madagascar and Mauritius.