It doesn’t make sense that every church is the right and true church. Some Christians believe the Eucharist is really the Body and Blood of our Saviour; other Christians claim that He was not God; still other Christians believe the God of the Old Testament is evil while Jesus is good. This doesn’t even bring in members of other religions where beliefs range quite widely, some are even outright opposed to Christianity.
It can make sense.
What you are saying is, “They cannot all be true churches if they believe different things.” Thus, you are assuming that, in order for a church to be a true church, it must believe the right things, and they cannot all believe the right things if they all believe different things.
First, let us deal with the assumption that a church must believe in the right things in order to be the true church. How can we establish this? Is believing the right things really the requirement? Would God say to one person, “Well, you were not a good person, but you believed the right things, and so you are in”, and then, to another, “You were a good person, but you did not believe the right things, and so you are out”? Only an unjust God would do such a thing.
Even assuming it were true, think about the range of opinions between the members of any one church. Two Catholics might both believe in Papal Infallibility, while one believes in Limbo and another does not. Each human being has a slightly different belief set from the next one.
Thus, the true church, whichever one it is, must be composed of people who believe different things. If they have to believe what is true, then the true belief must be a shared core belief: I believe A+B+C, and you believe A+B+D, and we are both saved because we believe the necessary core beliefs A and B, while C and D are irrelevant.
Obviously, this same argument applies to different denominations within one religion, and to different religions.
At this point, we encounter a major epistemological problem: how can we, as mere mortals, actually know Almighty God’s basis for salvation?
Personally, I am quite fond of the Catholic response to this: the Catholic belief that
other churches can be saved is an honest admission that they do not presume to know the mind of God. The Catholic church believes, in essence, that it is saved, but is too wise to pronounce upon the salvation of others.