Reviewed religious subject is to include earnest faster (8)

'faster' is the definition.
('speedy' can be similar in meaning to 'fast')

'reviewed religious subject is to include earnest' is the wordplay.
'reviewed' says the letters should be written in reverse.
'religious subject' becomes 'RE' (Religious Education).
'to include' indicates putting letters inside.
'earnest' becomes 'deep' (both can mean serious or solemn).
're'+'is'='reis'
'reis' enclosing 'deep' is 'reideeps'.
'reideeps' in reverse letter order is 'SPEEDIER'.