"Serious sin enters into our lives as we yield first to little temptations.  Seldom does one enter into deeper transgression without first yielding to lesser ones, which open the door to the greater. . .It is extremely difficult, if not impossible, for the devil to enter a door that is closed.  He seems to have no keys for locked doors.  But if a door is slightly ajar, he gets his toe in and soon this is followed by his foot, then by his leg and his whole body and his head, and finally he is in all the way.
This situation is reminiscent of the fable of the camel and his owner who were traveling across the desert sand dunes when a wind storm came up.  The traveler quickly set up his tent and moved in, closing the flaps to protect himself from the cutting, grinding sands of the raging storm.  The camel was of course left outside, and as the violent wind hurled the sand against his body and into his eyes and nostrils he found it unbearable and finally begged for entrance in to the tent.
'There is only room for myself,' said the traveler.
'But may I just get my nose in so I can breathe air not filled with sand?' asked the camel.
'Well, perhaps you can do that,' replied the traveler, and he opened the flap ever so little and the long nose of the camel entered.  How comfortable the camel was now!  But soon the camel became weary of the smarting sand on his eyes and ears, and he was tempted to ask again:
'The wind-driven sand is like a rasp on my head.  Could I put just my head in?'
'Again, the traveler rationalized that to acquiesce would do him no damamge, for the camel's head could occupy the space at the top of the tent which he himself was not using.  So the camel put his head inside and the beast was satisfied again--but for a short while only.
'Just the front quarters,' he begged, and again the traveler relented and soon the camel's front shoulders and legs were in the tent.  Finally by the same processes of pleading and of yielding, the camel's torso, his hind quarters and all were in the tent.   But now it was too crowded for the two, and the camel kicked the traveler out into the wind and storm.
Like the camel, Lucifer readily becomes the master when one succumbs to his initial blandishments.  Soon then the conscience is stilled completely, the evil power has full sway, and the door to salvation is closed until a thorough repentance open it again."                                        The Mirale of Forgiveness