I'm pretty sure the definition of those terms goes something like this:
When you are presented with a problem, or question that you do not understand the answer to, or something you cannot prove, you have two courses of action:
Doubt or Belief.
A Christian at some point in your life presented to you the idea that you are dead to sin, and that your current course will lead you to Hell, which is eternal separation from God (or eternal punishment as a lot of preachers will call it), and that the way out of this was to accept Christ and Repent for your Sin (along with all of the other things that come with that).
You had two courses of action:
Doubt (which is what most people do)
Believe (which is what we did)
Now, when you Believe in something that you cannot prove (God went to great lengths to make sure a lot of this stuff could not be proven with the methods and knowledge of Men), you believe because of Faith, not because of Proof.
We believe in gravity, because we have proved it exists. We don't know why it exists, we just know that it does, and it has a certain strength, and it affects objects in a certain way. There is no "Faith" in Gravity, because we've Proven it.
We believe that 2+2 = 4, because we've proven it. There's no Faith there.
We believe in God, and the things He has commanded us to do, even though we cannot Prove that He exists using our means (He designed it this way on purpose). That's Faith.
Now, no Christian will ever be perfect. Satan constantly throws attacks at us, left-and-right. He is constantly accusing us, trying to make us feel wretched and unclean, trying to tell us that God doesn't love us, and at times, he tells us that God doesn't exist and we should forget about the idea of Heaven because it doesn't exist.
Wherever your weakest point is, is where Satan will try to hit you the hardest. But, sometimes he'll throw a changeup because you'll be expecting your weak point to be hit. If your weak point is, say, sexual sin, he might decide to throw a Doubt in God's Existence at you because you'll be expecting and preparing for a sexual sin attack.
The point is, every Christian will have doubts at some point in their lives. Maybe when you go to bed, you might think "What if I'm wrong, what if this is all wrong? What if He doesn't really exist? What if when I die, I just cease to exist?"
That's Satan talking, not you. Push it out of your mind. Pray. Ask the Lord to rebuke Satan for you, maybe even say a quick apology if that helps you feel better. The most important thing, is that despite any doubts you might have in a day, He will forgive you and gladly welcome you back after you fight it off. He knows Satan is constantly attacking us, and He knows that each time Satan tries and fails to topple us, we build our bases stronger to prepare for the next attack.
He knows there will be doubts. We are imperfect, and we are still imprisoned in our flesh. I'm pretty sure that 100% Faith and 0% Doubt is kinda like the "sinless" life -- it is simply impossible to attain while on this Earth. Doubt is actually a sin itself and since a "sinless" life is impossible to attain, one can easily say that 0% doubt is also impossible.
The important thing is, that you fight the doubt off and you remain steadfast on the path. Stumbling once in awhile will happen, just stay on the path till its end. Even if you veer off the path, yell out for the Lord and He will come and find you and lead you back to the path.
The Good Shepherd won't lose a single one of His sheep, as the Bible says.
EDIT: I'd also like to remind the OP of "Doubting Thomas". One of the APOSTLES doubted in Christ's resurrection, despite Christ telling them this would happen beforehand (and of course, don't forget Jesus's reaction to Thomas's doubt). If an Apostle doubted, what does that say about us? Of course we can't have perfect faith. That's just impossible.
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