The older son represents the Mosaic law.
"Now the older son … became angry, and when he refused to enter the house, his father came out and pleaded with him. He said to his father in reply, ‘Look, all these years I served you and not once did I disobey your orders; yet you never gave me even a young goat to feast on with my friends. But when your son returns who swallowed up your property with prostitutes, for him you slaughter the fattened calf.’" (Luke 15:25-30)
The reaction of the older son represents the Mosaic law, whereas, in the Parable of the Prodigal son, Jesus introduces the law of the new covenant. The older son is rightly angry, as he has obeyed the law his whole life. The father understands that, and begs the older son to come inside, so he can gently explain why he has welcomed the younger brother home.
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