I'm going to give a slightly different understanding of this than you may have heard before, largely because I've faced up to the same thing and wondered the exact same thing. The question comes down to this: "Isn't believing in something simply the same as doing something? Isn't it a work in and of itself?" This question always bothered me.
Here's the viewpoint I tend to take:
It's not the fact that we believe or don't believe that saves us, it's the fact that we believe or don't believe that indicates that we are saved.
See the difference? In one, the belief causes salvation, and in the other, the salvation causes the belief.
My understanding of this is guided by Ephesians 2:8-9:
For by grace you have been saved through faith. And this is not your
own doing; it is the gift of God, not a result of works, so that no
one may boast.
What is the "this" that Paul is referring to? I would argue that the "this" is referring to the faith more than it is referring to the "saved" or the "grace".
Our faith, in and of itself, is also a gift of God. He's the one who enables us to hold the faith.
Of course, there are people who would say that this viewpoint is equally offensive, because it removes much of our personal autonomy in deciding our eternal fate. When you take this viewpoint to it's ultimate end, it's offensive because it's GOD who decides who is and isn't his, not us, and this is a terrifying thing. It puts us at the mercy of God, and not God at our mercy.
This is expected. Paul even addresses this in Romans 9:19-21:
You will say to me then, “Why does he still find fault? For who can
resist his will?” But who are you, O man, to answer back to God? Will
what is molded say to its molder, “Why have you made me like this?”
Has the potter no right over the clay, to make out of the same lump
one vessel for honorable use and another for dishonorable use?
People will say this is offensive, and they'd have to reject the whole concept based on this. Again, expected (1 Corinthians 1:18):
For the word of the cross is folly to those who are perishing, but to
us who are being saved it is the power of God.
But Paul maintains that it is God who chooses, and not us: (Romans 9:15-16)
For he says to Moses, “I will have mercy on whom I have mercy, and I
will have compassion on whom I have compassion.” So then it depends
not on human will or exertion, but on God, who has mercy.
Jesus himself doesn't even pray for everyone in the world when he prays in the Garden of Gethsemene. He prayed for those who are God's specifically, making a clear distinction between those who are, and those who are not, part of the Kingdom of God (John 17:9)
I am praying for them. I am not praying for the world but for those
whom you have given me, for they are yours.
These are just a few examples in scripture of where you can see this kind of thing. Once you start seeing a few, however, it's hard to not find more.
On a personal note, I haven't always held to this viewpoint. There was a time when I thought that if I believed, then God would save me. I took the belief-first viewpoint, and ultimately, it began to frighten me. I would question my salvation, and it would often come in the form of these questions: "Do I believe enough?", "Is my faith strong enough?", "Is my faith truly faith, or is it works?" I could never say that my faith was strong enough, because I found that faith wasn't simply something I could "grow". It was either there, or it wasn't there, and it seemed to grow on it's own regardless of my efforts to make it grow.
When somebody had shared the viewpoint that the faith comes from our salvation, and not the other way around. It started to make sense, but ultimately it simply raised another question: "Am I really chosen?" Doubts come in all flavors.
Ultimately, I found that this question is different in that it puts the burden of assuring me of my salvation on God, and not on myself. This makes sense, because I am not the agent of my salvation, God is. So it would only make sense that he who saves is the same one who assures of the salvation.
Technically, this viewpoint is called Calvinism, but personally, I call it life-changing. I did, however, have to come to the understanding that God is not a machinated, feelingless automaton, but rather a personal God who, out of his grace, has chosen to save me, and has given me assurance thereof.
Hopefully, this post will not only be good "debate-fodder", but also be something to ponder personally. I know many Christians that I love who would disagree with this viewpoint, and I believe that God still loves them, and I plan on seeing them in heaven with me, but I also see this viewpoint as being much, much easier to swallow, and much more consistent with the nature of salvation, than the "faith causes salvation" viewpoint.
Also, as an addendum: I want to address the "Good person" fallacy. According to Christianity, regardless of your viewpoint on Calvinism, the "good person" doesn't exist. Paul himself says, in Romans 3:10: "There is no one righteous, no not one."
In other words, we're all on the same level before God: bankruptcy. Therefore, we all need his salvation.
So we need to clarify. In Luke 18:19 and Mark 10:18, Jesus says, "Why do you call me good? Nobody is good except for one--God". If we're talking about a good person, are we talking about God? Because scripturally, that's the only one we could be talking about. Our goodness is not the issue here, because "all our righteous acts are like filthy (lit. menstrual) rags". (Isaiah 64:6)
To understand any of the Gospel at all, we have to start with the fact that we are primarily fallen and sinful, not primarily good. For many people, this is ultimately what they find offensive, because we want so badly to be able to say that we're good people, but the Bible simply does not give us that luxury. In fact, it does everything it can to remove that luxury as far away from us as possible.
As Steve Brown would say, the two points are this:
- Cheer up...you're a lot worse than you think you are, and
- cheer up...God's grace is a lot bigger than you think it is.