It's been said that you should avoid discussing religion and politics because it inevitably leads to arguments. If he knows that his parents are on a completely opposite end of the spectrum from you, your boyfriend should clue you in. But what if he doesn't? Or what if Dad asks who you voted for in the last presidential election, or Mom asks where you go to church? If your answer doesn't meet with their approval, it can be a tricky situation. Even if you can figure out what the "right" answer is ahead of time (that political sign in the front yard, for example), avoid the temptation to lie. If your relationship with your boyfriend is long-term, it's going to come out eventually.
Be honest about your opinions, but try to do in a way that is respectful of their beliefs. If they say something that rubs you the wrong way or even offends you, stay composed and change the subject, or say something like, "I'll guess we'll just have to agree to disagree on that," if necessary. We hope your boyfriend will help you out; you might consider having a pre-arranged word or signal to let him know that you need assistance.