In Sunday school, we looked at the Mosaic Covenant. The difficulty is how to understand the complex web of both gracious promises (“I will surely save you”) and conditional language (“obey and live; disobey and perish”). Because on the face of it, they seem at odds. Either God relates to us by grace or by works. It can’t be both.
The answer is that the Mosaic Covenant is fundamentally of grace. This is Paul’s argument in Galatians 3:17, when he says the Mosaic law does not cancel out the promises of grace given to Abraham. So also, in Exodus 19:4, God reminds his people that they were brought out of Egypt on eagles’ wings. JRR Tolkien was inspired by this biblical imagery in The Lord of the Rings, when giant eagles come to rescue Gandalf at Isengard (see picture to right). The point is that when you are being carried on eagles' wings, you are doing nothing, contributing nothing. And so the Mosaic Covenant is essentially of grace.
However, in the Mosaic Covenant, there is also law. Obey and the people will flourish in the land; disobey and the people will be banished into exile. As Paul argues in Galatians 3:24, the Mosaic law was a tutor to teach us that we cannot stay in the Promised Land (Eden) on the basis of our obedience, but that we need a savior, Jesus Christ, who would fulfill the law for us as our substitute. This is the point of the whole drama of redemptive-history. The law cannot save - only Jesus can save. This is the gospel according to the Old Testament.