Many miss the following, simply because they do not study The Old Testament Books as thoroughly as they do The New Testament Books.
God first gave The Gospel of Jesus Christ to and through Abraham. Apostle Paul refers to this in Romans 4 and Galatians 3, that The Gospel was preached to Abraham.
Abraham believed God, and it was counted to Abraham as righteousness. Apostle Paul likewise it is the same with all those of Faith, that righteousness is accounted by Faith on God's Promise, The Gospel.
Melchizedek, which Hebrews 7 reveals was Lord Jesus Christ in Old Testament times, met Abraham and offered him "bread and wine", sacraments of The New Covenant. Hebrews 7 even goes into the matter of Melchizedek blessing Abraham, even before there was a Levitical priesthood.
Many of our Sunday School teachers have misled us, because pointers to The Gospel of Jesus Christ exist in just about every Book of The Bible, starting with the end of Genesis 3. King David was given to prophesy about Christ's crucifixion about a 1,000 years before it happened (see Psalms 22). Isaiah was given to prophesy about Christ's death also (see Isaiah 53). There are also many OT prophetic references to Christ's future coming, gathering of the saints, and His future reign.
To discover much of this, it's like the Bible student has to go through the whole New Testament study first, and then go and backtrack in The Old Testament in-depth, instead of just jumping through it with short topic history like in youth Sunday School.
Not only do the firstborn of the children of Israel have a literal birthright they inherit from their flesh father, but our Heavenly Father Himself has a Birthright that He gave through His chosen elect. Per Old Testament history, that Birthright from GOD Himself begins with Abraham. Thus it's important to notice anytime God gives a blessing or promise through the Old Testament Patriarchs, because often it is pointing to His Birthright.
An Example:
1. To Abraham, God promised that he would be the father of a seed as many as the sands of the sea and stars of the sky. He also promised Abraham that he would rule over the gates of his enemies (great military strength). God also included blessings of inheritance of the land. Apostle Paul said The Gospel was preached to Abraham.
2. Then to Abraham's son Isaac, many of those same Blessings from God are given.
3. With Jacob, he is given the new name ISRAEL to represent the 'overcomers', i.e., the believers of The Gospel of Jesus Christ. The seed blessing given again, plenty of corn and wine (i.e., best land resources), etc.
4. Then with Jacob's son Joseph, he was blessed while separated in the land of Egypt, and was exalted to second ruler. He had two sons while there, Manasseh the elder, and Ephraim the younger. 1 Chronicles 5 reveals God's Birthright finally wound upon Ephraim and Manasseh. But Judah would receive the royal sceptre of kingline. With Ephraim, that is where God's Birthright stopped, and is still today with the tribe of Ephraim, the head tribe and largest one over the ten northern tribes that were scattered out of the holy land first, and never to return as a majority, even to this day. Thus they are called the Lost Ten Tribes of Israel.
5. God then reveals prophecy of what He would do to Ephraim and the ten tribed "house of Israel". That He would scatter then to new lands, and there give them the full weight of their Baal idol worship they forget Him for. But then He would give them a door of hope, and a new covenant in new lands, and no longer call them, 'not My people' (Lo-Ami), but instead Ami, meaning 'My people', and others with them would also be called the sons of the living GOD. (see Book of Hosea).
6. Apostle Paul in Romans 9 would quote from the Book of Hosea to Gentile Roman believers on Jesus Christ, referring to them also as the sons of the living GOD reference in Hosea along with the ten tribe "house of Israel". All this about the "house of Israel" (ten tribes) being inheritors of The Gospel along with believing Gentiles in new lands is about the foundation of the early western Christian nations that began to put away their Baal idols in Asia Minor and Europe, and instead began to accept The Gospel of Jesus Christ on national scales, Great Britain being the first nation to do that per history.
Much is there in the Old Testament prophets about The Gospel, and it's a simple matter of taking the time to go thought it and recognize those things Christ and His Apostles taught pulling from it. Even Apostle Peter and Jude revealed that Enoch, the 7th from the man Adam, knew about Jesus Christ and His future 2nd coming (see Jude 14).