As mentioned earlier by JMD, many of the founders of early tarot were also staunch Catholics (even families whose members included popes!). I'm not a Christian myself, but I found Christian symbolism very important when developing my own tarot deck.
There are legends and archetypes that have universal attraction, that are part of our (Western) combined cultural memory - some of these symbols appeared in decks such as RWS and Marseilles, and I brought in more elements into play as well when I was constructing my deck. St Francis of Assisi, the Garden of Eden, God Creating the Animals, the Nativity etc - you don't have to be a Christian for these stories, whether you believe them to be literal or metaphorical, to have resonance.
I hope that the Christian iconography that I've used helps Christians to use the deck in a way that feels right for them (say, for contemplation and prayer). For the rest of us, it's an acknowledgment of the part these symbols play in the development of modern society, regardless of our personal religious beliefs.
In regards to Islam, hmmm... perhaps I should try making a deck without people in it? I like a challenge!