Can you explain how someone can suddenly believe something that only minutes before, they required proof for? How does one go about making themselves believe the unbelievable?
As far as believing in your heart goes, which I think you are asking about, in Christianity we call it "grace". It's a free gift of God that cannot be forced. It's like that first moment you realize you are falling in love, the flash of inspiration that scientists often have.
But you can also believe in the will without the heart.
I am a skeptical person by nature, so I analogize it to the scientific method. Before you run an experiment, you have to have a hypothesis. You can believe the hypothesis or not. You can be trying to prove it or disprove it--doesn't matter. But if you abandon it and then don't run the experiment, you can never ever find out the truth. You have to run your experiment as if the hypothesis were true, then see how the data shakes out.
How do you suddenly believe? You say, ok, I dunno if there's a God or not, but I am going to find out one way or the other. You run the experiment. You say "Ok, let's suppose that God DID exist. Then what? Who is He? Has He communicated with man? Does He still?"
A little less than 20 years ago, as an agnostic, I ran that experiment. I am happy to call myself a Christian today.