Question: "Why did Jesus say to cleanse the inside of the cup (Matthew 23:26)?"
Answer: Jesus came to the nation of Israel proclaiming the good news that the kingdom of the heavens was at hand (Matthew 4:17), because the King (Jesus) had come. He came teaching them about the eternal kingdom of God that was in the heavens and that would one day change addresses, moving to earth in fulfilment of God’s promises. Jesus used many examples and illustrations, and in one instance He challenged His listeners that it was important to “first clean the inside of the cup and dish” (Matthew 23:26).
As Jesus proclaimed the coming kingdom, He urged the people to repent, or to change their mind about how they could enter that kingdom. In part because of the teachings of the Pharisees and scribes, many of the people thought that they could enter the kingdom of God based on their lineage or based on their external obedience to the Law of Moses. Jesus strongly contradicted that thought, challenging them especially in His Sermon on the Mount (Matthew 5—7) that external obedience to the law did not constitute righteousness. They needed internal righteousness, and they did not have that internal righteousness. Their righteousness had to be greater than what the Pharisees and scribes were prescribing, or else they could never enter the kingdom (Matthew 5:20). Instead of pursuing their current course of simply trying to look like they had righteousness, Jesus wanted them to look to Him to provide them with the true righteousness that they needed. It was important to cleanse the inside of the cup (Matthew 23:26), Jesus would later explain. First cleaning the inside of the cup was so important, in fact, that Jesus compared those who thought they would enter the kingdom through external obedience to a man who built a house on the sand. Those who looked to Jesus and discovered true righteousness were like a man who built a house on the rock. When the inevitable storm came, the house built on the sand fell, but the house built on the rock stood fast (Matthew 7:24–27).
The Pharisees and scribes thought they were righteous, but Jesus called them hypocrites who did not have righteousness and who were misleading others down a path of false righteousness (Matthew 23:13). He exposed their hypocrisy, saying that they were stealing the houses of widows while offering long public prayers to make themselves look holy (Matthew 23:14). He pronounced woe on them for making disciples of themselves and their error, instead of following Jesus as they ought to have done (Matthew 23:15). They were claiming to be guides, but they themselves were blind. They were misrepresenting God and His path to righteousness (Matthew 23:16–22). They were tithing, bringing glory to themselves, but were neglecting justice, mercy, and faithfulness (Matthew 23:23).
Jesus illustrates the spiritual bankruptcy of the religious leaders by saying that they focused on cleaning the outside of the cup and dish but were full of robbery and pursuit of self-interest (Matthew 23:25). Jesus says they should first focus on cleansing the inside of the cup and dish (Matthew 23:26). If they do that, the outside would be clean as well. Jesus further illustrates the problem of hypocrisy by calling them whitewashed tombs—they were pretty on the outside, but they were full of death and decay on the inside (Matthew 23:27–28).
Jesus’ message is consistent. To make a cup truly clean, it is important to cleanse the inside of the cup (Matthew 23:26). That is, it is necessary to have authentic, inner righteousness that only Christ can provide. The Pharisees and scribes were pursuing righteousness by their own efforts, but they failed to recognize that even our most righteous deeds are like filthy rags (Isaiah 64:6). We have nothing to offer God that would make us righteous. The only way we can be righteous is faith in Him. This has always been the recipe, as evidenced by the early example of Abraham, who was declared righteous when he believed in God who was speaking to Him (Genesis 15:6). In the same way, we are justified (declared righteous by God) through faith in Him and now have peace with God (Romans 5:1). Because of this we are citizens of His kingdom (Colossians 1:13) and will one day see Him in all the glory of that kingdom (Revelation 19—20). It was true when Jesus first said it—it is important to cleanse the inside of the cup—and it is still true today. Righteousness is on the inside by faith in Jesus.