Common sense. Here's some data...
Administrations must be adapted to the time periods in which they are carried out. The administration with Adam before the fall was different from the one with his immediate family after the fall. The administration with Israel “under the law” was carried out on different principles from the present administration of Grace. This present administration is different from the one that will characterize the return of Christ. The administration of Judgment will be different from the one that will belong to the administration of Glory, when all things shall be gathered together in one under the headship of Christ.
We will never understand the truth of God’s Word if we neglect to rightly divide the subject matter. As far as we are concerned in this Grace administration, what happened to Israel in the Old Testament was written for our learning. If we do not rightly divide to whom it's addressed—the Jew, Gentile, or the Church of God, we will use one truth to contradict another truth, and we will use what is true for one group in contrast to what is also true for another group.
These different administrations are suited to different times because God has spoken everything to its proper time and administration. We will never understand the truth of God’s Word if we read into one administration what God tells us belongs to another administration. If we believe what God said in one administration and carry it into another administration that was on a different principle, we will be taking what is true for one time, and using it to contradict what is also true for another time. When we mix them all together, by jumbling the whole Bible together: Law, Gospel, Grace, Judgment, Glory, Jew, Gentile, and the Church of God, we will be very confused in our understanding of the truth of God’s Word.
What is written directly to the Jews, belongs to and is for the Jews. What is written directly to the Gentiles, belongs to and is for the Gentiles. What is written directly to the Church of God, belongs to and is for the Church of God. What does God mean when He tells us that the visions shown to Isaiah was concerning Judah and Jerusalem? It was not addressed to us or written concerning us, but it was addressed to and concerning Judah and Jerusalem. It would be dishonest for the Church of God to interpret to the Church of God what God said concerns Israel.
The present administration of God is in the time period of the New Testament known as Grace. It deals with the new covenant, and it belongs to the time that is called the administration of the mystery. It's a period in time that was not made known to any one prior to this administration because God kept it a secret since the world began. From this our Grace administration, we learn God’s secret purpose that He had placed in Himself, according to the administration of Grace, which was first revealed to the apostle Paul.
From the eighth chapter of the book of Romans, it's written to those who live in this present Grace administration, “There is therefore now no condemnation to them which are in Christ Jesus” and “that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.”
From the sixth chapter of the book of Deuteronomy, it was written to those who lived under the Law administration, “it shall be our righteousness, if we observe to do all these commandments before the Lord our God, as he hath commanded us.” However, from the third chapter of the book of Romans, it's written to those who live in this present Grace administration, “by the deeds of the law there shall no flesh be justified in his sight.” What was written to those who lived under the Law administration is the complete opposite of what is written to us who live under the Grace administration.
We will always be in darkness and confusion regarding the truth of God’s Word if we do not understand the different administrations in the Bible. All hope for our redemption is in Jesus Christ, who was born into this world, died, and in the resurrection he became the head of a new creation. The living resurrected Christ Jesus has become the one great subject that occupies the Word of God that the church belongs to. It's this Christ Jesus that is the key to the divine revelation in the Word of God for this our Grace administration. The contents of the New Testament must be understood in reference to Christ Jesus our Lord because the doctrine and nature of God for this our Grace administration are centered in His Christ.