These troublesome brothers in Book 1 are seriously lacking. Their names literally mean "without faith" (Sansfoy), "without joy" (Sansjoy), and "without law" (Sansloy), so it's no wonder they aren't the most fulfilled trio you've ever met.
In the text they're identified as "Saracens," which is a term usually applied to Muslims, but could mean anyone in general who didn't believe in Christianity (and during Spenser's time, people were not particularly tolerant of this). Because these brothers aren't Christians, Spenser imagines that they therefore must be lacking, lacking the true faith, and the joy and rules that come with in.
Without these three qualities, the brothers play by their own rules, chasing women and fighting with knights. For Redcrosse, their main antagonist, grappling with these brothers represents a confrontation with religious doubt.