How Were Old Testament Believers Saved?

On Sunday, we wrapped up a sermon series answering a handful of questions submitted by the inquisitive folks of Alliance Bible Church. I've been working on addressing more of them here. One question asked is one I've heard a number of times: "How were Old Testament believers saved?" Mark Dever answers this question succinctly.
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To expound this a bit further, In Romans 4, Paul argues Christians are saved by grace not works. He quotes Genesis 15:6, "Abraham believed God and it was counted to him as righteousness." What did Abraham do to receive this gift of righteousness? He believed God. But what did he believe? 

In Galatians 3, we're told God "preached the gospel beforehand to Abraham..." Throughout his life, Abraham was shown the gospel. He was promised his descendants would be as numerous as the sands on the seashore. He was promised that through his family line all nations would be blessed. He was shown God's salvation through a sacrificial substitute through the testing of his faith with Isaac. Abraham entered into a "right" relationship with God by faith in God's promises in general and climactically in the gift of a Savior.

What we Christians have been given that Old Testament believers did not have are specifics and details about the Savior. This is both a gracious gift and daunting. As there is a "ratcheting up" of God's revelation to us, there is a "ratcheting up" of accountability to it. As there is a "ratcheting up" of God's specific gift of grace in Jesus Christ, there is a "ratcheting up" of culpability to do something with it. 
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