My party has a Barbarian who multiclassed to wizard. Levels 3 and 1, respectively.
He has incorporated the Staff of Defense (from Glasstaff in the Lost Mine of Phandelver Starter Set adventure) into the handle of his great axe and is attuned to it. He also has Shield in his spellbook.
When raging he loses his ability to cast his Wizard spells, but can he use the Staff's ability to cast Shield as a reaction while enraged?