I feel like religious texts are the only way to define it. If people who identify as Christian but do not practice any of the virtues of the bible define Christianity, then the word loses all meaning.

"Vegetarians eat meat all the time, because I have several self-identifying vegetarian friends who eat chicken, pork, and beef regularly!"

The culture surrounding Christianity is certainly league ahead of the one surrounding Islam at the moment, and has been for a while granted (though this was definitely not always the case), but I would place that solely on the shoulders of secular culture surrounding those various religions (which is pretty much non-existent for Islam, granted).