Biblical dispensations are periods of time in which God, having revealed His truth to man, holds man responsible for obeying it. Failure to do so brings God's judgment, and with it, a new set of instructions. In order to understand the dispensations of the Bible, one needs to be mindful of both the text and the context. For the most part, the text reveals God's point of view; He is either directing or correcting those with whom He is working. For example, God told Adam what he could and could not eat. Then, He corrected him, disciplined him, and gave him "new marching orders." In this case, the context consists of God's relationship with two people in a garden. God's instructions to Adam do not apply to the rest of humanity, because none of us have lived as innocents in God's garden. However, it is true that man can learn valuable lessons from the mistakes or poor choices of others, even though there were under a different dispensation. We are told in 2 Timothy 3:16-17 that all Scripture is valuable for a Christian's growth.
Today, Christians don't go out to find manna to eat. We don't offer animal sacrifices for our sins. We aren't expected to possess the "Promised Land." We are not required to be vegetarians or eat kosher foods. The dispensation known as the Church Age is unique from all other dispensations. By accepting Jesus, we have been given new understanding (light). It is as though God has a lamp with a seven-way light bulb. As the dispensations progress through time, He turns up the light one level until His whole truth is clearly seen. In each dispensation, God's people are to be stewards of the light God has given to them.
The best example I know of how failing to keep the dispensations separate hinders our understanding are the teachings of Jesus, is in Matthew Chapter Twenty-four. When His disciples asked Him when the temple will be destroyed, what will be the sign of His return, and what will be the sign of the end of the world (better "age" or dispensation), Jesus responded to the second and third questions, but not the first. They should have known from Daniel 9:26 that it will occur after He has left. The second and third question are not in the order they will occur. Jesus answered the third by describing the Tribulation in Matthew 24:4-28. He next taught them of His return in Matthew 24:29-46. Because the Church is not present on earth during the Tribulation, the passage does not speak of the Rapture of the Church. The Tribulation has absolutely nothing to do with the Church. Those taken away, are taken for judgment, and those remaining are allowed to enter into Christ's kingdom when He returns.
Today, the false teaching called "Replacement Theology" is widespread throughout Christendom. It says that since the Jews rejected their King, God has now applied the promises to the Church. One need only read Romans Chapters Nine through Eleven, to see that Israel was blinded in part so that Gentiles could be saved. When the last member of the Body of Christ believes, the time of the Church Age will be complete, and God will begin Daniel's Seventieth Week of judgment upon Israel. They will repent and the rest of the prophecies concerning Christ's Kingdom will then be fulfilled. The dispensations of The Law and The Church must be understood separately to prevent misinterpreting God's Word.