A philosopher once gave this axiom to arrive at the fact that he really did exist: "I think; therefore, I am." He could not say: "I'm just imagining that I exist" -- for as soon as he included the "I" in his statement, he was admitting that a self existed.
A person's selfhood is one of the most profound things in the universe. It should humble us and overwhelm us to think that we are individuals, separate from others. We are islands in many ways. No one can feel your exact feelings or think your exact thoughts. They are solely yours. They are inextricably part of you and you alone.
Within this part of our humanity ("self") lies that which is of utmost importance to God: our self-will. The will is that part of us which is truly ours, and truly us. Imagine our lives are like throne rooms, and our will sits on the throne over our own little universe. When outside information comes to the attention of our will, we do with it what we please. That is, we have a choice to make. And because it is our universe and our will is on the throne, the choice is final.
So in comes some information from another part of our universe, while our will is seated on the throne. "Dear king," we hear, "it has come to my attention that you will suffer greatly if you eat that third piece of pie." This is our conscience speaking, and we can either ignore the information or listen to it and heed it. "Away from me, you silly fool," says the will, "I'll walk around the block after supper and be just fine."
This is an important concept to understand, for it has everything to do with how we regard God. At some point in our lives, it's likely that our conscience brings us some "outside" information concerning God. It might go something like this:
"Dear king, it has come to my attention (and I certainly wanted to alert you of it) that someone called God is seeking entrance into your universe. He says that he has free reign to go anywhere he likes, but that he will not come into your universe unless expressly invited.
"He also says that if he comes in, he will probably have great influence over the decisions you make on your throne. I really am sorry, but that's what he says. But, says he, if you do this, your universe will become a better place. He says he is loving, and only wants what's best for you -- which is Him.
"He gave me the impression that he, unlike yourself, if you don't mind me saying so, is some sort of perfect King. He says you can trust him, better than you can trust yourself. So, what say you? Would you like to give him permission to enter?"
Haven't we all experienced something like that? God lets us make the decision. That is the great dignity with which he treats us. He does not barrel into our universe. He does not overthrow our monarch. Instead, he asks to come in. In his own words, Jesus said, "Behold, I stand at the door and knock. If anyone hears my voice and opens the door, I will come in."