Theologian isn't commonly revered in stopping case of sacrilege (5,9)

'theologian' is the definition.
(I know that Saint Augustine is a Church Father)

'isn't commonly revered in stopping case of sacrilege' is the wordplay.
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'revered' becomes 'august' (synonyms).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'case of' means to remove the middle letters (outsides of).
'sacrilege' with its centre removed is 'se'.
'aint'+'august'+'in'='aintaugustin'
'aintaugustin' put inside 'se' is 'SAINT AUGUSTINE'.