It is often thought that the Old Testament presents salvation by works, while the New Testament presents the opposite, salvation by grace.
Not true, however – God’s way of salvation is always by grace and it has always been so. Instead, the New Testament only shows how God makes His grace available to all – through Christ’s work on the cross.
The best place to get a good understanding of how salvation came to Old Testament believers is Romans chapter 4. It happened that the sect of the Judaizers were saying that salvation came through faith in Christ and obeying the law, i.e., the command to be circumcised. But Paul shows that Abraham was saved when he believed God, and that happened before he was circumcised. He was not saved, in other words, because he was circumcised, or because he kept the law…
For what does the Scripture say? “Abraham believed God, and it was counted to him as righteousness.”…We say that faith was counted to Abraham as righteousness. How then was it counted to him? Was it before or after he had been circumcised? It was not after, but before he was circumcised. He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well, and to make him the father of the circumcised who are not merely circumcised but who also walk in the footsteps of the faith that our father Abraham had before he was circumcised. For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith. Romans 4:3, 9-13 (ESV)
This wonderful truth comes into focus in an interesting place in Revelation 15, where the saints sing a particular song…
And they sing the song of Moses, the servant of God, and the song of the Lamb, saying, “Great and amazing are your deeds, O Lord God the Almighty! Just and true are your ways, O King of the nations! Revelation 15:3 (ESV)
The saints sing a song of deliverance, and it happens to be the song of Moses, and the song of the Lamb.
And this points to another place we find salvation by grace in the Old Testament – God’s deliverance of His people from bondage in Egypt does not happen because they are worthy or because they have served Him faithfully. Apparently, they hadn’t. No, it happens because He is a God of grace. It is only after they are delivered that He gives them His law. In the same way, we believe in Christ and are saved, and then God tells us how to live.
So the song of Moses is the same as the song of the Lamb – it is a high note of praise from a thankful people toward their gracious and good God.