How David came to be unchangeable? (6,2,5)

I believe the answer is:

carved in stone

I'm a little stuck... Click here to teach me more about this clue!

'how david came' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'be unchangeable?' is the wordplay.
'be' becomes 'carved' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'unchangeable?' becomes 'instone' (I've seen this in another clue).
'carved'+'instone'='CARVED IN STONE'

'to' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app