"Their customs and beliefs"
The most standard translation of the Annals of St. Bertin narrates on Pepin that:
"The Northmen got to Clermont where they slew Stephen, son of Hugh, and a few of his men, and then returned unpunished to their ships. Pippin [of Aquitaine], son of Pippin, who had changed back from being a monk to become a layman and an apostate, joined company with the Northmen and lived like one of them (rirum eorum servat) (Annals of St-Bertin, a. 864, the translation is taken from [Nelson (trans.) 1991: 111])"
Pippin's alleged conversion/ apostasy had once been advocated by Wallace-Hadrill in 1970s (as a kind of counter to the first wave of the revisionism in Viking studies around 1960s and 70s, represented by Peter H. Sawyer's Age of Vikings), but his reading is not so conclusive: Since ritus(<ritum) also/ rather mainly mean "way of life", almost synonym to mos ("way of life" in Latin). Nelson also annotates that Pipin is mentioned here and the scene of his capture in the entry of the year 869 as an "apostate" probably not because he abandoned Christianity, but because he abandoned the monastic way of life and returned to the lay society, meddled with the Vikings (Nelson (trans.) 1991:111, note 5).
Wallace-Hadrill's (as well as Alfred Smyth's) view of emphasizing the religious craze of Viking raid in the 9th century West, represented by the notorious ritual execution of "Blood-Eagle", has now largely been abandoned by any serious researchers (cf. Coupland 2003).
Works mentioned.
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(Edited): fixes some mistakes in format.