WE BELONG TO OUR LORD, that is, our Owner, just as the farmer's sheep belong to the farmer; that is why we carry his mark. If you have had anything to do with sheep, you will know that they have an incorrigible habit of squirming their way through hedges and getting mixed up with the sheep of the farmer next door. And that is why, especially when sheep are turned out to graze on hillsides, the farmer who owns them puts a sort of splotch on their sides, rather like what happened to you the last time you spilt the red ink. And so, if they do get straying and mix up with other people's flocks, there's no great harm done. So it was that, when you were baptized, our Lord put his mark on you, the sign of the cross. You and I can't see it there, because it belongs to the supernatural, not to the natural order; but an angel can see whether you are baptized or not, just as easily as you or I can see when somebody's got a smut on their nose. And that mark is indelible; it never comes out.
- Ronald A. Knox