Here is an analogy to go along with Scripture. Imagine you are a plate.

The plate is covered in greasy butter (which represents sin).

You cannot put the greasy plate into the cupboard because all the dishes in there are clean. A greasy plate isn't allowed, no matter how much it is wanted to be placed in there. (The grease represents the fact that we all sin.)

Good people think, "I am sure if I am good enough, I can rinse this sin off with my good works". (That effort to draw close to God by being a basically good person is represented by us rinsing the plate in running water. But, no matter how hard we try, water will not mix with the grease enough to lift it off.)

The solution is to apply dishsoap. Soap is part grease and part water, so it links with the grease and lifts if off the plate. (This is representative of how Jesus is fully God and fully man. Only he can lift away the sin. He offers to do this for us. But, we have to apply the soap...repent of sin, desire to turn from it and ask Jesus to be our Savior.)

I just wonder why people want to keep their plate greasy when the soap is sitting right there, and we didn't even have to purchase it. It is a gift. This simple analogy isn't perfect, but it works well enough, I think. I am thankful to Jesus for paying the price for my sin while he was God and while he was human. Only he could do this.