Hi there.

Julius II did indeed wear armour. He was elected in 1503 and his papacy was characterised by a great deal of warfare in the Italian Wars. Erasmus wrote negatively about Julius partly for this reason. You can read his (fairly withering) remarks in 'Julius Excluded From Heaven'.

There is a text about Julius II by Shaw, but any broad work on the popes, like Norwich or de Rosa, will give you an overview of the period and his exploits. His foreign policy in areas like the Romagna and Bologna have him the title 'the warrior pope'.

Damn.

I remembered a good painting of a Cardinal in armor, and I have found it, but the painting is not at all contemporary to the event.

Anyway, for what is worth here it is: Cardinal Richelieu at the siege of La Rochelle, 1627–28.

Unfortunately, the painting is from 1881, more than two centuries and half after the fact, I can't judge how realistic and well researched the depiction of Richelieu armor is.