Don't Go There
Never degrade yourself or your beliefs -- no matter how different they may be from what your in-laws believe.
While it may not be the primary cause of all family feuds, religion ranks in the top five reasons for family friction and the crumbling of relationships. What can you do if you find yourself half of an interfaith couple? How can you demonstrate that you're more than who you pray to? Here are some ways:
United we stand, divided we fall.
First of all, it's essential that you and your spouse present a united front. You and your beloved must explain to your families, in private, that you chose your mate. Cat got your tongue? Try this line: "If you love and respect me, you'll need to respect my choice and my mate."
Show interest in your in-laws' faith.
For example, if you're Catholic and your in-laws are Jewish, attend their Passover seder. (Watch the matzoh balls; a bad matzoh ball makes a good paperweight. They repeat like episodes of I Dream of Jeannie.)
Invite them to learn more about your religion.
Difference is scary; knowledge lessens fear. When your in-laws see that you don't handle snakes or sacrifice raccoons, they are likely to feel less threatened by your religious beliefs (unless you do handle snakes and sacrifice raccoons. Thanks for not sharing).
Whatever you believe, recognize that none of your in-laws is under any obligation to worship as you do. They have to respect your beliefs, not follow them. So share and share alike, but don't insist that yours is the one true faith. No proselytizing, please. No tracts on pillows, no earnest sermons.
Try to avoid confrontations.
Religious beliefs are a funny thing; you either have them or you don't. And if you do, nobody's going to sway what you deeply hold to be true. You won't get anywhere by antagonizing your in-laws. Here are my big three no-no topics: Abortion, birth control, sexual practices.
One couple I know were married in a Unitarian church, because, being Jewish and Episcopalian, they sought to find some middle ground of civilization where family members on both sides were less likely to kill themselves or each other. They selected that particular Unitarian church because on the day they checked out the church they were dedicating a pew to a cat. This leads to the final suggestion...
Try a little humor.
As you've probably figured out by now, I think a little humor goes a long way to defuse tough and touchy situations. But keep the humor light and playful; never snide or sarcastic. And never make one of your in-laws the butt of a joke-no matter how much they deserve it.