This is an often-asked question and I will seek to give an answer without going into much detail.
God already has a plan that was settled in eternity. This plan will not change. The scriptures is that plan written down. We see from the scriptures how God revealed His great plan a bit at a time.
Though God never changes how he deals with men and nations has changed as His plan was revealed. If all you had were Genesis then you would not have all that God has revealed. You would know of Creation, the Flood, the Calling of Abraham, and how Jacob?s family ended up in Egypt but there is more. During this time God gave commands ? He told Adam and Eve to put on skins and get out of the garden. He told Noah to build an ark if he wanted to survive. He told Abraham to believe some land would be his and he would be the father of a great nation. Basically God promised Abraham his descendents would have a big plot of land some day but the story doesn?t end here.
But then God gave you Exodus and more of His plan was revealed for now we see Israel made a nation and given the law. God had a plan for Israel and that was they were to be a nation of kings and priests to dispense God?s blessings to the rest of the nations in God?s eternal kingdom. Israel would be fulfilling this plan from the land God promised to Abraham back in early Genesis. From Exodus until Matthew we see Israel?s continuing failure to be what God called them to be. During this time God was giving the prophets visions regarding this future kingdom and a coming Messiah to bring in this eternal, earthly kingdom. When Israel finally went into total rebellion God turned them over to captivity and then brought them out again. Starting with Exodus 19 you are under the Jewish law.
Now, notice this whole time the church, as we know it is non-existent. God knew there was to be the church; the body of Christ but God was not ready to reveal this body yet for He was still dealing with Israel as a nation.
When Jesus shows up in Matthew he is basically there to ?bring in? this kingdom that was promised by God to Israel. Jesus came to Israel to bring them together for the future kingdom. At this time the Old Testament law is still in effect though it is in our ?new Testament?. Jesus comes to bring in this kingdom and is rejected just like God planned it. We know now that Jesus had to die for the sins of the world but up to that time this fact was basically hidden from all. We know this plan now but back then it was hid from them.
So, Israel rejected their Messiah and lost their kingdom offer. During Acts this kingdom offer is renewed one last time ? they reject it. Israel rejected God the Father in the OT, the Son in the gospels and now the Holy Ghost in the Acts period. So, is God?s plan finished? Of course not, God raises up a converted Pharisee, Saul, who becomes Paul and starts to reveal the body of Christ which God had planned from before the foundation of the world ? i.e. eternity. Paul is the one God chose to bring us this new plan. His epistles tell us of this new church, which was hidden from the OT folks but now, revealed to those in that day by Paul. When God is finished building His church then this age will end with the church being pulled out and He will resume His dealings with His chosen people, Israel during the Tribulation. At the end of this Tribulation period God will make the believing remnant of Israel that he brings through this tough 7 years to be His true people by putting His law into their hearts and making them anew and then bring the nation into their land to start the kingdom He promised to Abraham and the Jews in Exodus 19. Israel will finally be the nation of kings and priests to dispense God?s blessings to the other nations in the kingdom. They will rule and reign from Jerusalem.
What about the church that left before tribulation? The church of today (the body of Christ) will still be in the heavenly places displaying the manifold wisdom of god in heaven.
Now, why all that? We have the completed plan in our hands in a King James Bible to marvel at God?s infinite wisdom and grace. What we can do is look back and see how worked out this plan starting at Genesis 1 and finishes up in Revelation 2.
Right division tells us what instructions and commands went with each age. Right division tells what people and the instructions for those people go with what age that is being written about. From Gen. 1 to Exo. 19 you are dealing with folks and God?s instructions for them. After the law is given you are dealing with Israel and the instructions God gave to them. The Gospels carries on with these instructions. In Acts you have a transitional period where God is going from Jew to the church and the instructions, which go with that period. And now Mid Acts or so until Philemon you are dealing with the church, which is Jesus? body and the instructions for it which we get from Paul.
So, right division tells us what is for us and what is for them which is Israel and other folks in other times before and after our time. What you need to know is what is doctrinally for you today and what is doctrinally for the other folks in other ages. What you do not want to do is mix all those instructions together. That is why there is so much confusion today ? the applying of instructions meant for others to us today and trying to make instructions for us fit other folks in other ages.
Moses, David, the Prophets, gave us those instructions for the OT and the law. The Gospels gave us those instructions for the coming kingdom and other teachings about Jesus. Acts is a transitional book from the Jewish kingdom to the church. Paul gave us our instructions for us today mainly from Romans thru Philemon. You have Hebrews thru Revelation for those future tribulation times.
Adam ? "Put on some clothes and get out of the garden!"
Noah ? "Build a boat!"
Abraham ? "Believe you will father a great nation!"
Israel ? "Keep the law and sacrifice a lamb!"
Gospels ? "Hey Jews, believe that Jesus is your messiah!"
Church ?" Believe on the Lord Jesus Christ and thou shalt be saved!"
Tribulation - "Hey Jews, believe that Jesus is your messiah!"
Kingdom ? "Hey folks your King is on the throne ? keep his commandments"
I trust this helps. Many will present this a different way and see things different but this is how I would introduce one to what is right division.
I?m sure I could have done a better job but there it is ? I?ll proof read it later so forgive all mistakes