Perhaps a quibble, but Catholicism does not declare itself to have a “monopoly on truth.” On the contrary, the church takes pains to find common ground with people of other faiths and try to build bridges of human solidarity in areas where we hold common beliefs. In this way the church has affirmed some aspects of Islam, Buddhism, Hinduism, Judaism and even paganism. Perhaps you meant to object to the fact that Christians always believe they are correct on points that conflict with other religions.
Think about THAT for a second. Is it REALLY so unreasonable and uncharitable to actually believe what you say you believe and not declare it to be some sort of vague uncertainty? Perhaps it would be if, in fact, one makes up his own religion. But when it is one that is understood to have been handed down for 2,000 years from God incarnate who came to earth to preach that faith, live the example and give his very flesh as an offering of love? That’s not unreasonable unless you assert that the very idea of faith (properly defined) is irrational.