The trouble with the Pharisees was that they fasted without understanding the real purpose of fasting. They fasted for entirely the wrong reason.
How clear a right understanding makes everything. The religious leaders among the Jews could not see this. The illustration Christ uses makes it all so obvious. When God gives light from his word to a darkened soul, the way ahead suddenly becomes so much clearer. At conversion, the one whose worship has been filled with blind religious devotions, suddenly sees these are not what God wants. We turn away from those things and regard them as part of a former life of ignorance. True religion is full of understand, and sees what is an appropriate response to what God is doing in our lives. The believer may fast because there may be times of trial and special prayer may be needed, but the Christian life is primarily a life of rejoicing in the grace of God and our salvation in Christ.