It might make a little more sense if you apply it to something else...
The police tell you that when the stoplight is red, you must stop your car, and that if you do not stop your car, you will get a ticket. Some otherwise perfectly nice person with a friendly personality, and all kinds of friends, who does nice things every chance they get... decides that this rule does not apply to them. They run red lights. They get a ticket. Or maybe someone gets killed.
Point is... it's like the last poster pointed out. On the face of it, it may SEEM harsh for God to say, "this is the only religion in which you can be saved, and everyone outside of it will be lost," but this is simply because we picture our nice neighbors and wonder if they possibly could deserve to go to hell "just because of that".
The thing is, though, that the minute you begin to deeply consider the situation in any OTHER light (eg, forgetting for a moment how wonderful this or that person seems to YOU), and you begin to consider everything in the light of objective truth, or from God's point of view, it immediately begins to seem less ridiculous.
A few points...
1) Reality. Unfortunately, it is just kind of what it is. 2+2=4 is true for EVERYONE, even if they don't want it, don't like it, believe something else, whatever... All of reality operates like that.
If God says, for instance, "Thou shalt not commit adultery" ... and the rest of the world says, "marry and divorce all you want" ... you've got a conflict there. But there's only one truth, one reality about the matter. If religion were just a dream or fantasy, we could just change the commandment to save our otherwise nice neighbors. But since God is a real being with His own mind (that we do not invent or make up), and since His law is only what it is, and says only what it says... we're all kind of stuck with it. Now if God says, "THESE are the rules by which you will get to heaven or not..." then THAT's how it is, even if we don't care for it much, or even if it's hard to follow. BUT... and here's the key point here... deciding to believe something other than the truth of the matter WILL NOT HELP ANYONE.
In other words, if Mr. Jones on his 8th marriage decides to stop believing that law against adultery will really keep him from heaven... Mr. Jones is just lying to himself. He's simply wrong. And he does himself no favor by choosing to lie to himself rather than believing in the truth. If he believes in the truth, he can change his bad behavior and TRULY save his soul. If he rejects the truth for anything else, he's probably going to loose his soul, because he shuts out the one REAL way of saving it.
There is only one reality about God, His law, His religion, etc.. These things are all real, not imaginary. Thus, the only thing we should accept about them, is the truth. True, we CAN (are able to) lie to ourselves and convince ourselves of just about anything else. But the question is, how does rejecting the truth for anything else REALLY help us?
If I say rejecting the Catholic faith will result in a person going to hell, people often find that harsh. As though the person saying it is just being mean, or "close-minded." But when you apply the same logic to any other situation, the absurdity just leaps out at you. If I know, for example, that at the end of a certain road, the bridge is out, and there's a gaping cliff waiting... who is going to call me close-minded if I go around telling people that there is? The fact is, there IS! Now people can tell themselves the bridge is there. People can call me names for telling them otherwise, and stubbornly insist that as long as they choose to believe in it, that bridge will be there. But we all know reality doesn't work that way. In the end, when the car hits the edge of that cliff, it's GOING to fall. That's not my fault. It's simply the way things are.
Likewise, God laid out these laws and rules by which we are meant to live and be saved. But only those REAL laws and rules, and only that PARTICULAR way of life is going to do it. You can call me close-minded for saying it, but... if that's how it is, that's reality. If we accept it, hard though it may be for us, and try to fight it out until eternity with ourselves trying to follow it... we CAN save our souls. If we lie to ourselves, and decide to believe something that ISN'T true, and we tell ourselves that we're really all right no matter what we do or think, or no matter how we want to live... we've just thrown away every chance we had of saving our bacon for real. When we get to eternity, only the reality will be waiting for us. Lies we tell ourselves aren't going to help us there. Only by knowing the truth about the situation can we take measures to truly save ourselves.
In the end, we all will face the same reality. But it will be REALITY that we face, not whatever/anything we just preferred to believe. If I want to win a game, I need to know the rules, how it's played, etc... If I want to save my soul, I need the truth. Anything false is not going to help me at all.
2) Justice. The fact is, if a group of people got together, and one of them stood up and said that one or all of the rest deserved to die for not doing such and such... everyone would be a bit peeved about that, I think. And rightfully. One human being has no right (outside of lawful authorities) to impose any such thing on another, equal human being. If religions were created by men and imposed by men upon other equal men... then there really WOULD be something messed up about setting rules by which this or that person burned for eternity (were it possible to inflict that on someone). But God is not our equal, and that makes the whole "religion" question a whole different ball game.
A major problem people have with religion today, is the idea of God punishing. Today, "punishing" is seen as purely evil. The father that spanks the child is called an abuser. The mom who won't let her underage teen drink beer, is mean. The parents who ground their daughter because she defied them and went to a party where she knew there would be drugs, are cruel. This juvenile picture of authority and punishment, regardless of the reasons or rights behind it, is not just common among 13-year-olds. It's pretty much the creed of the better part of human society today. At some point, "authority" became synonymous with "cruel tyranny" and discipline and punishment with "evil" and "abuse". So naturally, the idea of an all-good God sending ANYONE to hell just doesn't sit right with a whole lot of people.
But that's because they have almost totally forgotten Who and What God is, what we have done and deserve for it, and the true meaning or idea of the word justice... let alone PERFECT justice.
It takes forever to write it out but... here's one more time for the kiddies back home...
Picture the universe before anything but God was in it. Now imagine God, with all of His infinite perfection. He is perfectly good. Perfectly loving. Perfectly merciful... in short, He is all good, to a perfect and infinite degree. There is no evil, no negative thing about Him whatsoever. Moreover, He is perfectly sufficient unto Himself, and perfectly content in Himself. In short, this perfectly and infinitely good God did not need anything whatsoever to be happy or content. He had no need of any kind to create anything, even if that anything had been perfectly loving and loyal to Him from the first moment throughout all eternity. But, as one priest so eloquently put it as far as I can remember, "God looked out upon nothing (eg, the empty universe), and God had compassion upon nothing (eg, the universe)." And He poured out His goodness upon that "nothing," and the universe (as we know it), was made.
And then something strange and inconceivable happened. God made man. He made man, even though He knew everything, and saw all things, and KNEW what would happen. Even knowing that countless souls would defy Him... an infinitely grave offense, because God is infinitely good, and infinitely lovable, and because it is the most heinous sort of insanity there is, that anyone can look at infinite perfection and infinite goodness, and HATE it, and REJECT it, and DEFY it, and REFUSE TO LOVE it, and infinitely grave, too, because God is the CREATOR of man, infinitely above him in authority, dignity, and justice... even knowing that man would be THAT profoundly wicked... even knowing that man would rise up one day and "KILL" God (the Son), God made man.
Now it may be easy for some to confuse God's knowledge of our wickedness, for some kind of excuse for it. But the fact that God knew we would be that wicked, does not excuse our actually DOING it. God may have known we would be, but it was US, with our free will, that CHOSE to be. God's knowing it did not force us to DO those things. It was us who freely CHOSE to do them. God gave every man since and including Adam and Eve, the CHOICE as to what they would do, and whether they wanted to love and obey Him (as they rightly and justly and sanely should do), or whether they wanted to be wicked and insane and choose hell instead. That choice was absolutely real, and absolutely OURS. So there is no use trying to put the blame on God for having created us. God wanted us to be with Him, happy forever in heaven. It was MEN who wanted and chose to go to hell (by choosing badly) by their own free will.
Ultimately, God gave every last human being the chance and the choice and the ability, and all the means necessary to save their souls. The real, true and only means to do so. But many men have not chosen to accept the truth, or to use those real means. Rather, in addition to the infinitely grave crime of committing the least sin, they will go so far as to even reject the very truth about Him, His existence, His rights over us as our Creator, etc.. He went so far to make known His will, as to take human flesh, and die a horrible death because He loved and wanted to save us. But before He did, He taught and made known His will to His apostles, and founded a Church upon them, and gave them the keys to His kingdom, and commanded them to go and teach all nations, that way to heaven.
When you think of it that way, it's impossible to think that it could be called unjust for God to allow men to have the rewards of their choices. After all, He gave His own life to try and save theirs. If after all that He has done, and all He has taught, and all He has given us, a person can still turn away from Him, refuse to obey Him, refuse to love and respect Him and His law and His way... that person is putting THEMSELVES in hell. That is the truth of the matter. God does not need to thrust men into hell. When we face Him, we will know full well what we have chosen by our lives. If we have chosen well, we will surely go to Him. If we are not guilty of hell nor worthy of heaven, we will surely run willingly to the purging fires of purgatory in order to do so. But if, by our lives, we have defied, hated, rejected and acted so wickedly toward Him, there will surely be no part of hell far or deep enough away from Him to hide, to conceal the shame and eternal enmity that will exist in such souls.
They made up their minds against Him, and lived apart from Him in this world, because THEY DID NOT WANT HIM. Not enough to accept the truth and live as He commanded them to do, and not just as they felt like doing. They loved themselves and their own pride and pleasure and will more. And by it they chose to separate themselves from Him forever.
Too many people today want God to be (or believe sincerely that He is) like a "softy" father, who will "love" their children enough to let them get away with murder, and still spoil them rotten and pamper and coddle them every moment, without any sign of harshness or consequences or justice of any kind. But even between children and mortal parents, this is not love, nor justice, nor good, but the evil of allowing injustice, and allowing the children to destroy themselves, and set themselves up for misery and ruin. However God is not a merely mortal father, but our Divine Father, infinitely good and infinitely above us. The debt we owe Him of love and gratitude and obedience, is infinitely greater. And He is PERFECT Justice.
God has His will, and His law, and the religion He gave us to save our souls. He gave us the chance and the means to get to heaven. If we do not want them, but decide for whatever reason to reject them, that is our choice, not His fault. But the reality of heaven and hell, and the laws and rules by which we will go to one or the other, are a concrete, inescapable reality to which all of us are subject, regardless of what we tell ourselves in our own heads. Just as we cannot change this life by imagining something else, neither can we change that one.
The doctrine of "no salvation outside the Church," when it comes down to it, is not "mean" or "close-minded." It is simply the right acknowledgment that an all-perfect, infinitely good, infinitely superior and infinitely just God has created us, and given us the means to save our souls... and that only by those REAL means... GOD'S means, not ours... can we really save ourselves.
God never said that man could defy and ignore and reject Him as long as we have good manners and talk kindly to one another, or as long as we're "sweet". We cannot think, therefore, that just because people of other religions or sects are nice, kind, good-hearted, whatever... that they can then ignore what God really did teach, or what really IS true or real about Him, His law or His religion. When God gives a command, His true children will obey it. Defiance with the sweetest smile and the kindest words, and all of the manners in the world, is still defiance. Sin, however sugar coated, is still sin.