The Catharijneconvent is a former monastery which today houses a museum of religious art. As the website states it:
'Get acquainted with famous pictures, ancient manuscripts, books, statues, altar pieces, ecclesiastical robes and relics. This remarkable museum is housed in a medieval monastery close to Utrecht's Dom.'
What more can I say? I've never been inside but think I will be soon.
Adress: Lange Nieuwstraat 38
That's next to the St. Catharina church.