Being able to discern the faithful son is actually an act of self-indictment on the part of the leaders. If they can rightly choose as faithful the son who left his disobedience, surely they must also be able to discern in themselves their own lack of righteousness (Ps. 53:2–3) and need of the Savior. Yet they never acknowledge their guilt. Let us never be so confident in our own goodness that we fail to recognize our need of repentance and forgiveness.
For further study:
The Bible in a year: