How can the Catholic Church claim to be true when it represents such a minority position within Christianity?

As an argument against Catholicism and in favor of any other brand of Christianity, your argument proves too much. Protestantism and Orthodoxy are each far smaller minorities than Catholicism. If Catholicism is untrue simply because it is a minority within Christianity–although it is the largest of the minorities–then neither Protestantism nor Orthodoxy can be true either.

As an argument against Catholicism and in favor of some other world religion, your argument falters for the same reason: Each of them–Islam, Hinduism, Judaism, whatever–represents only a small part of mankind.

Maybe you argue against religion of any kind. The secularist or rationalist who objects to religion in general (and objects because of the range of conflicting claims to truth) falls into the same trap. His materialistic scheme of the world is a minority opinion–most people are religious.

With respect to specifically Catholic/Protestant disputes, it’s a stumbling block to Protestantism that the Catholic Church–which Protestants, if they’re willing to acknowledge it as Christian at all, perceive as one particular church within the wider Body of Christ–claims to be the true Church founded by Christ and therefore the Church to which all mankind, ideally, should belong. No other Christian church makes this claim.