"I applaud this initiative that seeks to oppose and react to the stupid prejudice according to which Pope Francis is only a practical man devoid of specific theological or philosophical formation, while I was only a theoretician of theology who understood little of the concrete life of a Christian today," Benedict wrote in a letter read at the presentation of volumes on the theology of Pope Francis.
Benedict, 90, has lived in near-isolation in the Vatican since he became the first pope in 600 years to resign and he rarely makes public statements.
Many conservatives have given the 81-year-old Francis a failing grade in his first five years, saying his policies have sown confusion and accentuated a feeling of crisis.
"This is a very elegant but clear way of Benedict distancing himself from those who exploit him in their battle against Francis," said Father Antonio Spadaro, a Jesuit close to the pope who has written several books on him.
"Benedict is saying 'I won't allow you to do this in my name'," Spadaro told Reuters.
Conservatives say that in his attempt to make the Church more inclusive and less condemning, the pope is weakening attention to moral issues while focusing on social problems such as climate change, economic inequality and migration.
In 2016, four cardinals issued a rare public challenge to the pope over some of his teachings on the family, particularly the possibility of letting Catholics who have divorced and remarried in civil ceremonies receive communion. The pope did not respond, and one of the cardinals has threatened to "correct" him publicly.
Last year, several dozen conservative Catholic academics and lay people issued a 25-page letter accusing Francis of heresy.
One problem most conservatives and liberals alike believe Francis has failed to stem is sexual abuse, which has rocked the church for decades - and since well before the current pope's election.
Francis has been under fire for initially defending a Chilean bishop accused of covering up abuse and then days later doing an about-face, sending an investigator to look into the allegations.
In his letter, former Pope Benedict disputed suggestions by conservatives that Francis' academic qualities were lacking, praising his successor as a "man of deep philosophical and theological formation" and praising an "interior continuity between the two pontificates".
But the conservative blog Rorate Caeli, which is highly critical of Francis, tweeted on Tuesday: "The only continuity is that both are baptised men."
(Reporting By Philip Pullella; Editing by Hugh Lawson)