All religions are exclusive, precisely because there is such a thing as the law of non-contradiction. If you believe anything at all, you cannot affirm the opposite and still belong. If you don't "believe" what they believe, you won't be a part of that system. To affirm that anything at all is "true", is to become exclusivist.
"Becoming" a Christian is not so much in affirming creeds as it is coming to an understanding of Christ and applying it to your life. Yes it is believing things, but it is also trust, faith, and relationship. Becoming a Christian is more about "placing trust" in Christ as your ticket to salvation, then it is about believing truth statements about him, although the two go hand in hand. You place trust based on the truth value of some statements, but the power is not in the statement, but in the truth of it in your life. As is often said, even Satan knows the truth value of many statements, that they are true, yet it doesn't get him saved. Simply knowing something, or believing something is true, is only half the battle. You've got to apply it to your life. You've got to live it. The Bible says "consider yourselves dead to sin and alive to God". This is more than just believing it as a truth claim, but this seems to suggest we aught live like it. To "consider" ourselves dead is more than just believing a creedal statement that we are, it is actually living like it. We all believe McDonalds food is bad for us, but we all continue to eat it. So either we don't really believe it's bad for us, or we don't care. How would God think it if we believe statements of truth about him but we don't care, when we believe things are true but we don't live like it, we don't jump both feet forward? Obviously this kind of belief isn't what God is after, not just a simple intellectual acknowledgment, but full participation.
Besides, I'd say Christianity is the most non-exclusive religion of all (if it can be called a religion). Yes Jesus said he is the only way, but this is only exclusive insofar as terms of salvation, it is NOT exclusive in who can believe or "join". In other words, it doesn't matter if you are 4 years old or 98 on your death bed. Doesn't matter if you are male or female, white or black or red or yellow, tall or short, rich or poor, powerful or meek. It doesn't matter how long you've been a believer. It is not an exclusive club for gentlemen in suits. There is nothing preventing people from joining based in its "exclusive" nature. So is it really all that exclusive? Who is it excluding, and why? Is the complaint that it is exclusive because people have to believe it to join? Well duh, you tend to have to believe what it is you want to join. You can't "join" Christianity, hoping for its promised rewards, while affirming Satanist creeds. Law of non-contradiction and all that. You wouldn't be "excluded" because of some personal flaw or inadequacy, there is nothing preventing you from dropping conflicting beliefs and embracing Biblical truths except your own dark heart. So if a person cannot embrace the truths of Christianity, it isn't because Christianity is exclusive, it's because of their own preexisting loyalties. Christ requires those same loyalties.
If there are expressed on earth 5 ways to eternal salvation and bliss, one cannot just believe them all and hope for the best. If they conflict, one has to choose where one's loyalties lie. "Choose this day whom you will serve." And one cannot "serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other."
You cannot place your trust in multiple people, or multiple systems, to save you.
In any event, being more or less exclusive speaks nothing about how true it is. Less exclusive doesn't mean more true, it might mean more comfortable perhaps. If one believes all people go to heaven no matter what, then that is a pretty good deal, you don't really have to "believe" anything. But that doesn't make it more true then an ultra-exclusive system. God may in fact only like women and so all men go to Hell, period. Sucks for men, very exclusive, but that doesn't make it less true. So arguing that something is exclusive really doesn't show anything anyway.
All that said, if you wonder if you are really a Christian, just ask yourself if sin really matters to you. Do you believe there is a creator God with a perfect standard and sin will keep you from entering his domain (Heaven)? If so, how do you get out of this sin problem? Do you earn your way out? Obey rules until death? Or do you accept that God sent his Son to "become sin for us", to take our penalty, so that "by his stripes" we can be healed? Do you believe this was done for you? That if you call upon the name of Jesus, he can forgive and wash you clean before God?
This isn't just truth statements that you say "yes... yes... yes..." to. It is trust, and faith. "Without faith it is impossible to please him". These truths you have to have embedded in your heart. These truths are all eggs you've put in one basket, your life is in that basket. Everybody can sit around talking about how bad fast food is even as they pass the Big Macs around the room.
If these truths mean something to you beyond an agreement with a creedal statement, then I'd say you're a Christian.
"So everyone who acknowledges me before men, I also will acknowledge before my Father who is in heaven, but whoever denies me before men, I also will deny before my Father who is in heaven."
One thing will certainly boot you from the Christian camp, and that is to deny Jesus. You could probably deny a whole mess of things (probably to your own regret), and still be Christian, but you can't deny that. There are very few "essentials", the rest is just how good your understanding of God is, and doctrine and theology. I'm sure many "ignorant" (willfully or otherwise) and rebellious will enter Heaven, but nobody can who isn't cleansed by the pure blood of Jesus.
None of us aught judge the heart, nor label each other saved and not saved, but I think it's pretty safe to label when it is so clear in scripture. I'm not to worried about labeling a person unsaved who denies Jesus and blasphemes God. But if you are the type of person who thought Jesus was just a moral teacher and it sucks he died, but we get to Heaven by going to church on Sunday and trying to "be good" as best we can, and shouldn't God accept us because we "gave it our best go"? Then I'd be worried you're not a Christian.
I would not worry so much, if the point of contention is just more liberal or more conservative points on doctrinal views.
Peace