Is Christian symbol infinite in alternative religion? (5)

I believe the answer is:

islam

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'infinite in alternative religion?' is the definition.
'Islam' can be an answer for 'religion?' (Islam is a religion). I'm not sure about the 'infinite in alternative' bit.

'is christian symbol' is the wordplay.
'christian symbol' becomes 'lam' (I can't explain this - if you can you should give a lot more credence to this answer).
'is'+'lam'='ISLAM'

Can you help me to learn more?

(Other definitions for islam that I've seen before include "Adherents of a religion" , "Mails to Muslim religion" , "Major world religion" , "Mails to great eastern religion" , "Religion of the Koran" .)

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

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