Opus Dei is a cult. Four of my siblings (who even as young children were faithful, practicing Catholics) went to Opus Dei schools. The message that was delivered was you either join us (become numerary and supernumeraries)and do everthing we say (wear horsehair undershirts, etc.) or you will be expelled. Needless to say they were expelled.
Sounds just like SSPX.
Well there we have it. An infallible pronouncement from Pope Grey Ghost II