The gospel for today really follows on from yesterday’s gospel when chief priest and elders are questioned by Jesus regarding John the Baptist’s authority. They could not answer Jesus out of fear. So the parable of the Two Sons answers them indirectly.
The sons represent two groups of people. In the context of the gospel they are sinners who repent, and Israel’s leaders who refuse the Baptist’s message (even when tax-collectors and harlots do!).
So the son that initially says “no” to his father, but later thinks better and actually does what the father wants him to do, are the sinners who repent. The other son who initially says “yes” to his father, but does not actually do what his father wants, are the chief priests and elders. They talk a good game, but they do not actually live it out.
All of us for whom our faith is very important need to be reminded of this teaching of Jesus. It is easy to say “yes” and not follow through, especially if we get caught up in a sentiment towards God. Then we might promise all sorts of things; whereas sentiments do not last, our promises to God do. Following through on our promises is hard work and we do not usually have the motivation of warm feelings all the time, yet it is of utmost importance lest we find that we actually deserve the title hypocrite!