The Pharisees were very concerned about position. They wanted to look good to men. They wanted the praise of men rather than the praise of God. But Jesus taught that it is not what is on the outside that matters but what is on the inside. That is the essence of this teaching to cleanse the inside of the cup first so that the outside would be clean too.
When you think about a cup, if it has been used you will always wash it out before re-using it. And when you wash it your focus will be more on the inside of the cup than outside, for it is the inside that will hold your drink. You do not want your drink contaminated by grime so you will always focus a greater attention on cleaning the inside first. Even if the outside were unable to be properly cleaned, you could always use a spoon or a straw to eat or drink the contents, but the most important thing is that the inside is clean first.
In this analogy Jesus uses the cup as a representation of men, specifically the Pharisees. His beef with them is that they will go to great lengths to clean up the outside but spend little or no effort on the inside. Back in Matthew 23:5 he said, “They do all their deeds to be seen by men; for they make their phylacteries broad and their fringes long.” They focussed on wearing the cloaks and clothes of their office as priests so that they would look “righteous.” They wanted people to look at them and say, “Look at him in those clothes. He must be a righteous man of God.” But contrary to a common saying, clothes do NOT maketh the man. Jesus did not go about in robes that made him stand out from the crowd. He did not were the clothes, cloaks and garments of the clergy. He just wore the same kind of clothes that the rest of the people wore.
In Jesus it does not matter what you wear or what you look like on the outside. The focus of the New Covenant is not about making us look good, it’s about training us how to be good. The New Covenant and the teachings of Jesus are aimed at transforming us into his image. They are meant to bring us to maturity in Christ in our spirit, not in the flesh. Jesus said, “It is the spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.” (John 6:63) If we are to have our spirit changed and if we are to receive life, then it is our inside that must be transformed. It is our spirit that must be cleansed.
The Pharisees were focussed on making a good show in the flesh. They were trying to look good on the outside. Paul also upbraided those who did such things saying, “It is those who want to make a good showing in the flesh that would compel you to be circumcised, and only in order that they may not be persecuted for the cross of Christ. For even those who receive circumcision do not themselves keep the law, but they desire to have you circumcised that they may glory in your flesh.” (Galatians 6:12-13) There were some in the early church insisting on converts being circumcised according to the law. But Paul opposed them and repelled such actions for circumcision is not necessary under the New Covenant. As he said, they were doing this so they could glory in the flesh.
This is exactly what Jesus was saying too. They were trying to cleanse the outside without first cleansing the inside. It does not matter if a person is wearing the cloth of the clergy or is circumcised. These things do not make you any holier. It is only by focussing on the inside and being set free from sin that you can come to the Lord. It is only by accepting Jesus as Saviour and learning his teachings that we may be cleansed from sin and set free from law. Then our spirits will be changed and this will work from the inside out. We will be cleansed within which will cleanse the outside as well.