Including how a sinner is saved--by grace through faith in Christ. I remember how I fell short on this basic truth early in college, when I would tell my roommate that it seemed like there was more of a works component to salvation in the Old Testament. I failed to see the essential unity of the Covenant of Grace. One of my soldier friends was tripping over this same point last night.
Hebrews 11 chronicles many of the OT believers as having walked by faith. But that is just faith--it doesn't say anything about Jesus Christ, right? Wrong. Faith must always have an object, and that object was the Word--specifically the promises concerning Jesus. In Heb. 11, it was the promises given that drove the believers to walk by faith. God through His Word painted pictures of a heavenly promised land, the resurrection of the dead, etc. And ultimately, that great cloud of witnesses was looking toward Christ, the author and perfecter of our faith.
In Galatians 3, we are told that Abraham believed God (described as faith that comes by hearing), and it was credited to him as righteousness (v6). Was it the faith itself that was credited to him? Nope. It was the object of that faith--Christ, and His alien righteousness. Gal. 3:10-14 then makes the point abundantly clear (in refuting the Judaizer heretics)--if you think that works of the law play a role, then you must keep the law perfectly, or else you're dead. The law does not give life, it only condemns. That is why Christ had to do two key things for us: live a perfect life in our place (2 Cor. 5:21) and bear the curse of our wicked life (Gal. 3:13).
The rest of Gal. 3 then tracks how the law and the promise operated in conjunction with one another throughout the OT. The law, first given in the garden (don't eat of the tree!) and later passed down through Moses, was not a stairway to heaven but an impossible standard, exposing man's utter and desperate need for a perfect law-keeper and curse-bearer. The promise granted God's people the assured hope that such a person from God would indeed come. In other words, the law drove our faces to the ground and the promise lifted our eyes back to heaven for hope.
No, God's people of old didn't know all of the specific dimensions of Christ and His work. They didn't know the exact dates, his height, the color of his eyes, etc. And of course, most men wouldn't believe in Christ anyway, as the law and promise ultimately posed a great offense to human nature: Ye shall not be as gods, but as men in need of the Gospel of God. For OT believers, they saw the shadowy form of Christ on the horizon, while we recall the more distinct form in the rearview mirror of our faith. In both cases--even in the days of Jesus--the sight portion didn't particularly matter. It was bending one's rebellious heart in submission to Christ by the Spirit's power.
Romans 3:
21 But now the righteousness of God has been manifested apart from the law, although the Law and the Prophets bear witness to it— 22 the righteousness of God through faith in Jesus Christ for all who believe. For there is no distinction: 23 for all have sinned and fall short of the glory of God, 24 and are justified by his grace as a gift, through the redemption that is in Christ Jesus, 25 whom God put forward as a propitiation by his blood, to be received by faith.
