Old Italian Mass held by church leader — one who’s from Scotland? (9)

'old italian' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'mass held by church leader one who's from scotland?' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'held by' means one lot of letters goes inside another.
'church leader' becomes 'pope' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'who's from scotland?' becomes 'ian' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'm' going inside 'pope' is 'pompe'.
'pompe'+'i'+'ian'='POMPEIIAN'

Can you help me to learn more?