In today's Gospel reading from Mark 13, Jesus told his disciples to be watchful and to be alert, for they did not know the time the Lord would come again. Jesus made it clear that he will come again, and this time to judge us. How do I know? Well, if there won't be any judgement, then we really don't need to be watchful at all. For those who have always been faithful to the Lord or are already in union with him, they are ready for anything. So this is really a warning to those who are not yet ready. Since those who do not believe that the Lord had come the first time will have no reason to believe that he will come a second time, there is no point in letting them know when the Lord will come again. Besides, keeping the time and day unknown will help separating the true believers from the true unbelievers.