Mat 12:43 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest, and findeth none. Mat 12:44 Then he saith, I will return into my house from whence I came out; and when he is come, he findeth it empty, swept, and garnished. Mat 12:45 Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. Even so shall it be also unto this wicked generation.

G.K. Chesterton once said "If you leave a thing alone you leave it to a torrent of change. If you leave a white post alone it will soon be a black post."

If, when we are converted, we do not fill the space which once was occupied by evil with good, we will soon be in a worse state than if we had not done anything.