What is Covenant Theology?
Covenant theology holds, in terms of its basic understanding of Scripture, that we should understand the Bible as portraying fundamentally two covenants: a Covenant of Works and a Covenant of Grace. In the Covenant of Works, God made a covenant with Adam in the Garden, according to Covenant Theology. Namely, if you obey me and follow me and resist eating of the tree of the knowledge of good and evil; if you stay away from that, if you don't eat of that tree and follow me in obedience, then you will ultimately receive life. Covenant theologians have seen this as something more than the life of Adam then. It is not just a continuation of his life in the garden temporally, but what we would speak of as eternal life. They propose that there must have been a probationary period in which this testing was undertaken. Had Adam passed the test (who knows how much longer it might have been; maybe two more days and the test would have been over; we just don't know), then he would have received eternal life because of his works. But if Adam failed the test, if he were to eat of the tree of the knowledge of good and evil in the garden, then we know from the text, in the day that you eat from it you will surely die (Gen 2:17). So death for disobedience; life, presumably a better life, greater than the life he had now for obedience.
Covenant theologians acknowledge that the first part of this, the promise of life for obedience, is not stated explicitly in Scripture. But they think that it is implied by the negative statement, "If you eat of it you will die." If you don't eat of the tree, then you would receive the gift of eternal life. If that is the case, then it must be something different than what you have now, and if that is the case, there must be a probationary period. There must be a time period after which this would be given. All of that follows from what they know to be the case; namely, there is command given that if you eat of the tree you will die. The other part of it is spin off from that. We all know that Adam failed the test and brought death upon himself and all of his progeny. Romans 5 tells us that in Adam all sin and deserve his death. So we learn from Paul in Romans 5:12 and following that all die in Adam's one sin.
In order to save sinners, God brings about another covenant. This is not a Covenant of Works because sinners could never work to make the payment necessary to satisfy a holy God on account of the offense that has been committed. The guilt is too great, and the offense is too serious. Another Covenant of Works (work it off now, pay your dues, pay off your debt) won't work for human beings, for sinners. God inaugurates, instead, a Covenant of Grace, whereby his Son will pay the penalty for sinners, and those sinners in exchange will receive the righteousness of Christ. It is quite a deal for sinners. We give Christ our sins and he gives us his righteousness. Double imputation is part of this understanding as well. Our sin is imputed to Christ, so he pays the penalty for our guilt and it is charged against him even though he doesn't deserve to pay it. That's what imputation means at that point; our sin is charged against Christ, and his righteousness is imputed to us; it is credited to our account by faith - justification.
How much of the Bible does the Covenant of Works cover, what does it span? The Covenant of Works covers Genesis 1, 2 and part of Genesis 3 where the sin takes place. What about the Covenant of Grace where sinners now cannot be saved by works? If they are going to be saved it has to be by grace? Genesis 3 to Revelation 22. The point of this is that it leads Covenant theologians, in the traditional understanding, to think in terms of the broad sense of the holistic nature of virtually all of the Bible, from Gen 3 on, which is most of the Bible. Basically, the whole Bible fits under this Covenant of Grace notion. This leads to, in Covenant Theology, a strong sense of uniformity throughout the Bible, that is a strong sense of continuity. There is one thing God is doing from the sin in the garden and on, that is, he is providing for human sin and saving the people. The Covenant of Grace spans both Testaments; it spans Israel and Church. In that sense, it leads to a unified sense in all of Scripture: Old and New Testaments together.