Peter amended tiny part of Bible giving reason to stick together (6,2,7)
I believe the answer is:
safety in numbers
'reason to stick together' is the definition.
(reason to stay with a group)
'peter amended tiny part of bible' is the wordplay.
'peter' becomes 'safe' (slang term).
'amended' indicates anagramming the letters.
'part of bible' becomes 'numbers' (Book of Numbers).
'tiny' anagrammed gives 'tyin'.
'safe'+'tyin'+'numbers'='SAFETY IN NUMBERS'
'giving' acts as a link.
(Other definitions for safety in numbers that I've seen before include "a few could be at risk?" , "Reassurance company brings" , "Salvation in holy book" .)