"'Take your evil deeds out of my sight! Stop doing wrong, learn to do right! Seek justice, encourage the oppressed. Defend the cause of the fatherless, plead the case of the widow.'" NIV translation
God is showing the people that true worship of Him is not about rituals. Here, the people are encouraged to put their faith in action by serving instead of just showing up at the temple and going through the motions. The same message is true for the church today as many are showing up on Sunday but that is the extent of their "worship". It didn't please God then and it doesn't please Him now!
"'Come now, let us reason together,' says the Lord. 'Though your sins are like scarlet they shall be as white as snow; though they are red as crimson, they shall be like wool.'" NIV translation
This verse is probably the most quoted of all of the book of Isaiah. This verse tells of the promise of our sins being forgiven through the blood of Jesus Christ.
"'If you are willing and obedient, you will eat the best from the land,; but if you resist and rebel, you will be devoured by the sword.' For the mouth of the Lord has spoken." NIV translation
These verses, although they are from the same passage as verse 18, are not quoted as often or well known. Simply put, these two verses help us to understand that, after our sins are forgiven, we are called to obey. If we obey, we will be blessed. If we do not obey, the sword of Jesus' mouth will devour us. (He will deny us on judgment day.)
"'See how the faithful city has become a harlot! She once was full of justice; righteousness used to dwell in her- but now murderers!'" NIV translation
The "faithful city" refers to Jerusalem and the fact that God has set aside the city to be holy. This verse is a lament of the fact that the holy city was inhabited by all sorts of false gods and religions. The same is true of Jerusalem today as it contains the Dome Of The Rock (Muslim holy site), the Wailing Wall (Jewish holy site), and various other "religious sites". The same analogy is true of local bodies of believers (churches) of today as they have turned their back on the very word of God and allowed all sorts of false teachings to sway people from the truth.
"'Your silver has become dross, your choice wine is diluted with water. Your rulers are rebels, companions of thieves; they all love bribes and chase after gifts. They do not defend the cause of the fatherless; the widow's case does not come before them.'" NIV translation
This passage describes the corruption of the city and its people as they have turned away from God. The city was chosen by God to be a beacon of light but they now seek to serve themselves instead of those that are less fortunate. This passage also describes our world today as it is normal and expected that everyone is out to get as much as they can for themselves. This is in direct contradiction to the word of God but it is a great opportunity for us, as Christians, to stand out and be the shining light that Jesus called us to be.
"Therefore the Lord, the Lord Almighty, the Mighty One of Israel, declares: 'Ah, I will get relief from my foes and avenge myself on my enemies. I will turn my hand against you; I will thoroughly purge away your dross and remove all your impurities.'" NIV translation
This passage consists of a warning as well as a promise. God wants us to be made pure as that is the only way that we can have fellowship with Him. This passage says that God will get "relief" by purging the impurities. We, as Christians, need to stop and think about how our sins grieve God. If we are willing, God will remove the sin. If we hold onto the sinful practices, we will be removed along with the dross.