Jacqui, I agree with you that we just do not know of all the good that so many secret Samaritans do.
Where Christians would be missed is in the field of voluntary work.
Many , like me, do little or nothing but certainly the big organisations, even the secular ones, are mainly staffed and run by people who would call themselves christians.E.g. Oxfam, Save The Children, Red Cross, Tear Fund, Salvation Army, Christian Aid....