Jesus was talking with the most senior religious officials (the Chief Priests and Elders of the people - Matthew 21:23). He asked them questions to drive them to understand their spiritual poverty, despite their social distinction and wealth. The question in this passage was in the form of a simple riddle about words and actions.
 
It was a familiar agricultural scenario. The father needed the family to help him work the farm, with different types of work at different times/seasons. Approaching the first son, the father instructed him to work in the vineyard that day: the boy was very reluctant and openly told his father that he would not obey. So the older man went to the second son who eagerly agreed to serve.
 
However, the first son had second thoughts; he repented and changed his plans. Instead of disobeying, he decided to obey ... and the job got done. The second son, although he had pledged himself to work as instructed, decided to break his promise and disobey his father's command. When Jesus asked the top clerics to decide which son had please his father, the answer was obvious - the one who repented and obeyed. Jesus said that the most sinful people who repented were like that and they pleased God, whereas it was the religious leaders who had broken their promise to obey God.
 
It is important to keep true to your word (unless you have said the wrong thing). But more important than the words is the real desire of the heart. The Lord sees the heart and longs that we will desire what is good, choose to obey and then be willing to confess the truth of our heart and take the right action. Anything less is not godly. To say the right things without desiring to fulfil your promise, is a lie; but to repent of having a rebellious heart which has refused to obey, is good and pleasing to God. Religion, of whatever form, which tries to mask disobedience by good words - is a foolish deception. Ask the Lord to give you a new heart and a right spirit ... and the words will follow.