The Roman Catholic Church believes it is the One True Religion. All other religions have some truth, but only the Roman Catholic Church possesses the Fullness of Truth. However, does not every religion claim the Fullness of Truth for itself? If a religion did not possess Truth, its followers would all switch to the religion possessing the Truth. That is the purpose of religious truth, to be absolute Truth. No religion would ever imply fallibility. For Catholics, the Pope is infallible. For some religions, the Bible is inerrant. All religions have something of this nature: scriptures, a spiritual leader, their deity/deities.
Religions feign possessing Truth. Theoretically speaking, only one Absolute Truth about God/gods, the creation of the universe, the afterlife, and the proper ethics can exist. Yet, there are myriad religions. One of two things must be true: either every religion on earth is wrong, except one. Or, every religion on earth is wrong. An examination of these two claims is in order.
Assume every religion on earth is wrong, except one. How does a person figure out which one is the True religion? Well, every religion is based off faith—strong belief without proof. To persuade someone using faith is ineffective, especially when the person in question possesses no faith or possesses faith in a different religion. For example, a Christian telling a Jew to believe Jesus Christ is the Messiah probably would not go too well. The two would probably walk away from the encounter as a Christian and a Jew. A Christian telling an Atheist to believe in God would end with a similar result. Faith is not a solid basis for persuasion. This fact complicates finding the one True religion.
Obviously, a person is welcome to research every religion in the world in order to judge which one deserves the title, The One True Religion. However, the task can never be accomplished. Religions are based on faith, not on reason. Therefore, it is impossible to objectively investigate the validity of religious Truth. Religion is a subjective truth masquerading as absolute Truth. Religion is only as true as the individual makes it. Religious truth and the faith of the individual are directly proportionate. Put simply, if you believe, then it is truth to you.