Q: There are lots of religions in the world. So how can a Christian presume that his religion is the only right one?
A: Short answer: A Christian shouldn’t presume that. His religion isn’t the only right one.
Slightly longer answer: If by “the only right one” a Christian is saying that everything his religion says is right and everything every other religion says is wrong, then he’s denying one of the fundamental claims of his own religion, namely, that Christianity is the 
fulfillment, not the negation, of the religion of Old Testament Israel. Furthermore, it’s just obvious that Christianity has a lot in common with both Judaism and Islam. And, in fact, Christianity has various teachings and practices in common with pretty much every other religion in the world: Christian missionaries have been building on those common features for centuries.
Even longer answer: First, let’s clear the ground a bit. Just because there are lots of opinions about an issue doesn’t mean that one opinion isn’t right and the others are wrong. A math teacher might receive a wide range of responses (= ”opinions”) on an exam, but she knows that only “x + 3” is the correct answer. You ask for directions in a strange town to the museum, and four locals give you four different answers, but the answers usually aren’t all equal in effectively getting you to your destination. So the mere presence of multiple opinions says nothing immediately about whether there is more than one correct answer—or even whether a correct answer is available at all.
Second, we can think of religions as maps and directions on how to best use the maps. They describe reality and tell us how best to negotiate reality. As such, religions that patently fail to describe reality accurately or to tell us how to negotiate it effectively fall out of use in favour of religions that do a better job.
We can also assume that religions that do work, at least somewhat, will make assertions about reality that overlap with assertions made on other maps. If we’re trying to walk from the western edge of Venice to the eastern, any decent map is going to include a description of the Grand Canal and of at least one of the very few bridges that cross it. So even the worst map that actually works—that anyone living in Venice will give you–will share at least some information with the best map possible.
So of course the world’s religions share various claims and practices with others. The world is what it is and living in it is done most effectively this way rather than that, so religions that approximate those realities are going to share a lot of the same claims.
Third, allowing then that more than one religion can be true in important respects doesn’t mean that all religions are equally good, nor that one religion isn’t the best of those available. If you actually had a map and a guidebook furnished by the founder, planner, builder, and ruler of the area–who also demonstrably has taken great pains to communicate with you as truthfully and helpfully as possible–then you’d be very glad to have such instructions and you would have good grounds to consider them the best available. You might even want to share them with people you care about.
That’s what Christians do when they preach the gospel. They say, “We are so thankful to have been given The Directions by The Maker. And they’re free! Come get them!”
Maybe there are better directions available elsewhere. If so, please tell us. We, like any other sensible people, want the best help we can get. But we hope you won’t be angry with us if we’re pretty enthusiastic about what we think is the best map and guidebook we’ve ever seen and we want to share it with you.
In fact, shouldn’t you be angry with us instead if we wouldn’t?
John Stackhouse, from his latest answer at Ask John.
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