The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “All the nations will be blessed in you.” (Galatians 3:8, NASB95)

Here is something interesting, imagine that we leave out the clause that begins “foreseeing that God..”  We would be left with “The Scripture preached the gospel beforehand to Abraham saying…” What was preached clearly in the New Testament was latent in the Old Testament, and it is contained in the Scripture [which both Jew and Gentile agreed was the Tanakh, or what we call the Old Testament].

In the context of Gal 3.8, this is exactly Paul’s point, both the Old and New Testament contain the good news that salvation is by faith in Jesus.  In the Old Testament it is somewhat hidden, or perhaps we could say that it is awaiting further revelation to be clear, but it is all there.  It was revealed in the person and work of Jesus.

This is the power of the Scriptures, they contain the good news, they preach the good news, by reading them you can understand the good news: You do not have to remain in your sins, Jesus died for them, he died for you, or as Paul will put it elsewhere:

It is true that through the sin of one man death began to rule because of that one man. But how much greater is the result of what was done by the one man, Jesus Christ! All who receive God’s abundant grace and are freely put right with him will rule in life through Christ.” (Romans 5:17, GNB)