In order to better understand the Bible you must see the Old Testament as the foundation of the New Testament; because all that is being said in the New Testament has been shaped by the Old.
Be it the Gospels, Acts or the letters; we deal with fragments of the Old Testament Scriptures. You must also take into consideration that any Old Testament quotations are loose and often used out of the original context to make a point. For example The Letter of Ephesians chapter 6 is a quotation of Isaiah 59:17 mixed to gather with fragments of the Book of Daniel.
You must also be anchored in logic, which Apostle Paul first establishes. For example: In Romans 13:1 Paul declares that God establishes all governments. Then we must follow logic, which comes from the One. Genesis 15:16 shows us time and Daniel also. Combine these with Paul's statements and Ephesians 6 shines with better light. God sets up authorities until their righteousness or iniquity becomes complete.