When Jesus said that His followers would fast, He also explain to them some do's and don'ts of fasting. It was not to be a outward show, but something done in secret and that they would receive a reward. In His promises, one of those would be that of the Holy Spirit being given to them, in talking about this we see that it would be those whom repented, turned from them public proclamtion of not being seem. Those whose faith would have works, not just a formal dead religious-ritual.