As this appears to be an oversubscription appeal, you need reasons for wanting or needing a place sufficient to outweigh the prejudice to the school if they are to take an additional pupil.
Questions are most likely to be about your reasons.
http://www.elevenplusexams.co.uk/appeal ... -school#c2I'm not sure how strong the faith argument is going to be. (Note the post from ouflak above.) The school is indeed open to all faiths, but clearly the admission rules give priority to baptised catholics, and the appeal panel may choose to do the same (it's entirely up to them to decide how much weight to give to an appellant's reasons.)
"Attitude" is of no relevance in appeals, and character references aren't needed. Have you read the Q&As? Lots of advice there.
"Start here! Essential reading".Extracurricular activities are unlikely to be relevant, unless you can link them to something important on offer at the school which is not available at the allocated school.