To be perfect is to be complete - or to have integrity.
In this instance Jesus is teaching his audience which was his disciples on the mountain here, He said near the end here, closing - Be perfect, therefore as your heavenly Father is perfect. As mentioned earlier, that being perfect is to be complete or to have integrity; There are only two options to choose from.
Is Jesus telling his disciples to be perfect in the flesh? Or to be perfect and complete in faith in Him?
Here, with the disciples Jesus with a person who came up to Him. From verse 16 the man probably of Jewish decent due to what he is enquiring, saying Teacher what good thing must I do to get eternal life? What a direct and straight forward question here in which this person was to the Messiah who was the very Word of God sent to set the captives free. In verse 17 Jesus responds, which I believe He (being the very Word of God) is referring towards his flesh with this man here saying Why do you ask me about is good? Which I believe is pointing about how with-in the flesh there is nothing good about it (though inside which people had yet to know what the Spirit of God, and the very Soul of the Word of God made flesh. You may differ.
Jesus goes on to state that "There is only One who is good. (meaning God - Jesus's Father). IF you want to enter life keep the commandments." Why is Jesus saying this, if he was someone who fulfills the law and the prophets? Because Jesus is pointing the man back to the commandments which by his birthright he had done and knew about having probably gone to the synagogues growing up from childhood.
Which the Man asked Jesus which ones are you talking about that I should keep? Jesus replies with you shall not murder, commit adulty, steal, not give false testimony, honor your mother and father, and love your neighbor as yourself.
The man states back to Jesus, All of these I have kept. What is it do I still lack that I may gain eternal life? So here the man replies, saying he has kept most of the commands that Jesus had just mentioned to him, in verse 18-19, looking at verse 20.
Here Jesus mentions being perfect again, to be complete, have integrity - “If you want to be perfect, go, sell your possessions and give to the poor, and you will have treasure in heaven. Then come, follow me.” Verse 21.
Jesus again here refers again to the material means - which this means is for a spiritual means too - responds saying that if he desires to be perfect, go sell your possessions and give to the poor, and you will have treasure in heaven. Thinking about this, to let go of our flesh in trying to feed it by possessions it is better to let go, and to give those are in need so that the man will have treasure in heaven.
Just as Jesus said to his Disciples in Matthew 5, how is it we can be perfect? He stated that it was to be perfect as their Father in heaven is perfect. Here to be perfect, the man needed to materially sell everything he had because he was wealthy. Jesus ends it by saying once he has done that, he will have treasure in heaven and should come to follow Him.
After the man had left, - in an instance you can see a mark here between old covenant and new covenant Jesus was brining forth. As the man - left onward (not sure what he did) but went back to his material ways that he first performed by the flesh, of perfecting in obeying the law to be made right with God because Jesus doesn't say here right out they he was right with God just yet, until the follow Him part in which He would be made seen right with God by faith. That is where being complete in Him is found and it is finished comes into play that brings forth new life to a believer in which they begin to follow after Jesus.
So why is it hard for a rich to enter into the Kingdom of Heaven? Because they do not see their flaws before a truly perfect and holy God. It is easier for a camel to go through the eye of a needle than for someone who is rich to enter into the kingdom of God. The question here becomes what does it mean to be rich? That could be rich in pride, status, placing oneself above others, not just wealth itself. Because it is okay for Christians to make money, and save it and use it as they will but not to be come prideful above others because of it. That is why it is so hard for a rich person to enter into the kingdom of Heaven.
After explaining this to them, the disciples Who then can be saved?
Jesus responded and looked at them, with this is impossible (through material means), but with God (through spiritual means) with Him all things are possible.
So what does it mean to be perfect is it by the flesh or by and through faith in Christ and resting in his works, and having relationship with the Father, to walk in integrity to be open and honest with God with our failures and to believe on the one whom God had sent.