In my own estimation, the greatest contribution that Christadelphians have made to biblical theology has been their understanding of the close relationship between the promises God made to the patriarchs of Israel (Abraham, Isaac, Jacob and David) and the gospel or good news preached by Jesus and the apostles.
Christadelphians have correctly highlighted how much the New Testament writers emphasize the patriarchs and the covenants of promise that they received. This is particularly evident in the teaching of Paul. For instance, preaching in the synagogue at Antioch, Paul addressed the congregation as "sons of Abraham's family" (Acts 13:26 NASB). Having described the events of the death and resurrection of Jesus, he then declared, "And we preach to you the good news of the promise made to the fathers" (Acts 13:32 NASB). Later, explaining his relationship to Judaism as he stood on trial before Agrippa, Paul declared, "And now I am standing trial for the hope of the promise made by God to our fathers" (Acts 26:6 NASB).
In Paul's epistles, the same emphasis comes across. After a lengthy analysis of the promises to Abraham in Galatians 3, Paul concludes, "And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise" (Gal. 3:29 NASB). And again, in Romans 4, Paul emphasises that the promise to Abraham "will be guaranteed to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all" (Rom. 4:16 NASB).
Now, it is not only Christadelphians who have grasped the importance of the promises. One only needs to pick up a book such as The Promise-Plan of God by Walter C. Kaiser, Jr. to see that there are also Evangelical scholars who have understood that the promises are the unifying thread that runs throughout Scripture. Where Christadelphians have differed, however, is in their insistence that comprehending the promises to the fathers is a prerequisite for salvation in Christ. The Christadelphian argument goes something like this:
C1. Salvation is by faith in the gospel of Christ.
C2. The gospel of Christ is none other than the fulfillment of the promises to Abraham.
C3. Therefore, one cannot understand the gospel of Christ without understanding the promises to Abraham.
C4. Therefore, one who does not understand the promises to Abraham does not have saving faith.
This, I believe, is a flawed argument with a thrust that is very different than Paul's. I would express Paul's argument as follows:
P1. Salvation is by faith in the gospel of Jesus Christ.
P2. The gospel of Christ is none other than the fulfillment of the promises to Abraham.
P3. Therefore, whoever puts their faith in the gospel of Jesus Christ becomes an heir of the promises to Abraham.
P4. Therefore, Gentile believers are heirs of the promises to Abraham, regardless of whether they keep the Law of Moses.
Observe that the first two premises are the same in both arguments. The difference is in the implications of these two premises. I would assert that premise C3 is nowhere in the New Testament, and consequently the conclusion C4 cannot be inferred.
By contrast, premise P3 is central to Paul's argument. In Galatians 3, Paul's central concern is not with the promises to Abraham per se, but rather with basis on which Gentile believers receive the Holy Spirit (Gal. 3:2 NASB). Paul uses Abraham as a vehicle in his argument. He emphasises that the gospel was preached to Abraham in the words, "All the nations will be blessed in you" (3:8), inasmuch as all those who have faith are blessed just like Abraham was (3:9). Thus in Christ Jesus the blessing of Abraham comes on the Gentiles, so that they can receive the promise of the Spirit through faith (3:14). This promise is contingent on faith in Jesus Christ (3:22). Whoever has faith in Christ Jesus is a child of God (3:26) and consequently becomes a descendant of Abraham and an heir of the promises (3:29).
A similar line of argument appears in Romans 4. As we read earlier, Paul writes there that the promises to Abraham's descendants are for those who have the faith of Abraham. The faith of Abraham does not refer to an explicit understanding of who Abraham was or what he believed. Abraham had an embryonic understanding of what has now been revealed more plainly through Jesus Christ. The key element of Abraham's faith was his belief that "what God had promised, He was able also to perform" (Rom. 4:21). This faith was credited to him as righteousness. Paul's conclusion is then unmistakable: righteousness will in like manner be credited to all those who "believe in Him who raised Jesus our Lord from the dead, He who was delivered over because of our transgressions, and was raised because of our justification" (4:24-25). This is none other than premise P3: whoever believes in the redemptive death and resurrection of Jesus Christ effectively has the faith of Abraham and is a joint-heir of the promises made to him.
Space does not allow us to proceed much further, but it is worth mentioning that most discussion of the promises to Abraham in the New Testament occurs when speaking or writing to Jews or concerning Judaism. There is no mention of the promises to the fathers, for instance, in the message Peter preached to Cornelius (Acts 10:34-43), or Paul's speech to the Athenians (Acts 17:22-31). Although an argument from silence, this is contrary to what one would expect if the author of Acts sought to convey that understanding these promises was a prerequisite for Gentiles to be saved.
Now, all of this does not mean that the promises to the fathers are irrelevant or unimportant to Gentile Christians. An understanding of the covenants of promise in Genesis, and indeed of the whole Old Testament, is highly profitable for any servant of Christ (2 Timothy 3:16). One who is ignorant of these things is weak in his or her knowledge of the Word, and Christadelphians have done well to emphasize the promises in their teaching. However, there is a world of difference between saying a person's knowledge is limited and saying that such a person's faith is void.
What is wrong with the Christadelphian stance that a person cannot be saved without understanding the promises to Abraham? Firstly, it creates an unnecessary fellowship barrier. Secondly, and more seriously, it imposes a requirement for salvation the apostles did not impose. It comes dangerously close to violating the principle of Acts 4:12, by adding a second name (Abraham's) by which we must be saved.
Mercifully, God does not refuse to justify us due to flaws or limitations in our understanding of his Word:
"Now to Him who is able to do far more abundantly beyond all that we ask or think, according to the power that works within us, to Him be the glory in the church and in Christ Jesus to all generations forever and ever. Amen." (Eph. 3:19-20 NASB)