Why is the Christian concept of Heaven depicted as the Pearly Gates?

Not to create any sort of religious flame war, because it is obvious to anyone familiar with me that I am not a Christian, but it puzzles me as to why Heaven would need gates? Who are they trying to keep in? or keep out, as the case may be? Surely if someone wanted to sneak into Heaven, God would stop them before they jumped the fence, wouldn’t He? I’d like to have a reasonable explanation here.