God isn't looking to catch us in the act. He's omniscient; he's already caught us in the act. We don't need to be ready for His coming so that we can fool Him into thinking that we've been good all along. For that matter, His coming should not be a matter of fear and guilt. Instead, we need to be prepared for His coming in the same way that we get prepared for the return of a loved one from a trip, or for a party, or for any other sort of special occasion.
We should be ready for God's coming with joy, not trepidation. This is what Advent is all about: preparing us for the great festivity that is Christmas, the celebration of Christ's coming into the world. Why wouldn't we want to be ready for this wonderful event?