A couple of years ago I read a suggestion from one of my favorite authors, Scot McKnight, that really had an impact on how I read the Bible. He suggested that it is really helpful to read the book of Genesis with the promise to Abraham in Gen. 12 in mind, and that the entire book could be analyzed on the basis of that promise. He may have even suggested reading the entire Old Testament in this light, which would also make lots of sense.
In Gen. 12:1-3 God promised Abraham that He would make him a great nation, that he would make his name great, that he would give him and his offspring land, and that he would be a blessing to all the families of the earth. Think of how these promises relate to today's reading from the OT (Gen. 38-40)-
-The tawdry story of Judah and Tamar is really a story about lineage, and illustrates the "great nation" aspect of the promise.
-The story of Joseph illustrates how Abraham's offspring would be a blessing to all nations, as everyone for whom Joseph works is blessed by the prosperity the Lord brings through Joseph.
-And of course, the Joseph narratives serve to explain how Abraham's offspring ended up in Egypt, a prelude to their inheriting the promised land.
The same principle can be applied to the New Testament as well, since it shows us the ultimate way in which Abraham's offspring blesses the world. And today's reading in Mark 15 is the capstone of that promise. Jesus, a descendant of Abraham through Tamar and Judah (Matt. 1:2-3), became a curse for us, "so that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we might receive the promised Spirit through faith" (Gal. 3:14).