The cathedral can be cool (except in mid-summer). Be prepared with layers if you are planning to spend extended periods of time inside.

Besides your favorite clothes and personal toiletries…

Consider taking:

Bible

journal (& small scissors and double stick tape for adding things to your journal)

voltage transformer & plug adapters for any electric devices you bring

travel alarm

umbrella

Consider leaving home:

as many electric devices as you can

You’ll need to have:

passport (put extra copies in your suitcase(s) in case it is lost or stolen)

airline tickets

visa/debit card and/orATM card
insurance card

international drivers license & regular drivers license if you are going to rent a car

Some people report liking to bring :

art supplies
stone-rubbing supplies: tracing paper, small brush, carbon paper, masking tape

camera & supplies (extra camera battery, lenses, extra film or cards, tripod, download cable, chargers)

gifts or cards used to express appreciation

small binoculars

travel iron
ear plugs

travelers checks (in euros)
copies of all official documents (passport, credit card numbers etc.) in case they get stolen

a list including emergency contact information including phone numbers of family/friends at home
photos of important people and places from home

In your carry on you may want:

all prescription medications
a change of clothing in case your luggage doesn't arrive promptly
basic toiletries including a toothbrush & toothpaste (within current liquid restrictions)
anything that will help you be more comfortable on a long flight

a journal
snacks (nothing liquid)