"'So I will come near to you for judgment. I will be quick to testify against sorcerers, adulterers and perjurers, against those who defraud laborers of their wages, who oppress the widows and the fatherless, and deprive aliens of justice, but do not fear me,' says the Lord Almighty." NIV translation
The people have not believed that the Judge is coming but God not only says that He is coming but gives specific examples of those that will be judged. He not only says that He is the Judge but also that God is going to "testify against" them.
"'I the Lord do not change. So you, O descendants of Jacob, are not destroyed. Ever since the time of your forefathers you have turned away from my decrees and have not kept them. Return to me, and I will return to you,' says the Lord Almighty." NIV translation
God continues to speak to his people as he reminds them that he has never changed. God chose them to be His people and He keeps his promises. If God changed and did not keep His promises, they would have been completely destroyed because of the disobedience of their fathers before them. Then, God repeats his promise to them that, if they repent, He will forgive. That is the same promise that we have today in Jesus Christ.
"'But you ask, "How are we to return?" Will a man rob God? Yet you rob me. But you ask, "How do we rob you?" In tithes and offerings.'" NIV translation
We may say that this is the Old Testament and therefore it does not apply to us. That is partially right as we do not have to keep any of the law to obtain salvation as that is only through faith in Jesus Christ. But, Jesus told us that, if we love Him we will obey His commands and they will not be a burden. Tithes and offerings, for us as Christians, should be a heartfelt remembering of what God has given us and a simple giving back the first portion to Him.
"'You are under a curse - the whole nation of you - because you are robbing me.'" NIV translation
The people had stopped giving in faith and so we see here that God tells them they are "under a curse" and not just the individual but the nation as a whole. They had not been faithful with what God had given them so God can simply not give them the harvest until they get in line with His will. This is true for us today as well. When times get tough, many Christians stop giving a tithe and offerings so they can "make ends meet" but this is not of faith. A tithe is the first portion of what God has given you and God has given everyone something (see Luke 21:1-4). Offerings are anything that God places on your heart to give over and above what you give through the tithe. Anything less is robbing God and, if you do that, quite simply your finances will be under a curse.