Over at his blog Fides Quaerens Intellectum Christian philosopher Johnny-Dee writes:

I think the objection goes like this: It is wrong for you to believe that your belief is true because it implies those who adhere to other religious beliefs are wrong. This objection is rife with problems in validity and soundness, but I’d like to ignore all of that for now. Consider what the objector is suggesting: Christians should have a belief that they do not think is true. [Full post]

Sometimes when a person expresses offense when Christians claim that Christianity is true (not just "true for me" but absolutely and objectively true for everyone) they may indeed object because they hold to an incorrect and inappropriate conception of tolerance, as I've commented on previously (see Tolerance and Stating Facts != Hating). But more often they are objecting because of a similar but subtly different reason, namely that they are making what I'll call a category error regarding religious truth claims.

When some people express offense that a Christian believes Christianity to be really true, they are conceiving of Christianity as being in the realm of personal (relative) opinion rather than objective truth. That is, they see choice of religion as being like choosing your favorite ice cream flavor: A person isn't "wrong" because they prefer vanilla over chocolate. So too, the erroneous argument goes, a person isn't "wrong" because they prefer Baha'i over Christianity. Greg Koukl talks about this using the ice cream / insulin analogy:

There is significant confusion on this point. Americans think of God, religion, and morals like ice cream and not like insulin. They choose religious views according to tastes, according to what they prefer rather than according to what's true. [Full post]

Of course, this raises the question of whether choice of religious belief is really like choosing our favorite ice cream flavor. Hmmm, after adding the picture above I really crave ice cream … Ahem. Like I noted in my article Aren't there many different paths to God? for From Today On (also posted here):

If someone is dying and needs medicine, you need to give them what will heal them, not what they like best. In the same way, Jesus gives us what we need, and ultimately what is best for us. There are many different paths, but they don’t all eventually lead to the top of the same mountain. Some veer off to the left and the right; others climb entirely different mountains! And if God is real, truth about God is not like ice cream; it’s like medicine, and only what is true can heal.

Further reading: Three Tough Questions and Their Answers by philosopher Michael Horner, including "Aren't all religions the same?"