15 To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. 16 Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. 17 This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. 18 For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise.
The language of the AV is somewhat difficult here, and the newer translations are more helpful. The phrase 'I speak after the manner of men' in 15 may be paraphrased thus (following J.B. Phillips): 'Let me give you an everyday illustration'. The point Paul is making is that once a covenant or contract has been legally drawn up and signed, it cannot be interfered with by any third party. Thus when God made a covenant with Abraham and his seed, it could not be affected in any way by the Law, which came into existence much later. If then the receiving of the promised blessing were now made to depend on the Law, this would amount to a cancellation of the original covenant or contract, which is both illegal and impossible. This is a weighty argument, designed not only to demonstrate the validity of Paul's doctrine of justification by faith, but also to prompt discussion on the function and purpose of the Law as such - hence the question in 19, 'Wherefore then serveth the Law?' The words in 16b are really a parenthesis. J.B. Phillips puts them in brackets, thus: (Note in passing that the Scripture says not 'seeds' but uses the singular 'seed', meaning Christ). Paul thus underlines the fact that the promise made to Abraham was fulfilled in Christ, and therefore Abraham's faith looked forward to what Christ was to do in the fullness of the time in His death and resurrection. Consequently it is only in Christ that we can know the blessings of the covenant, namely justification before God and the promise of the Spirit (14). Everything depends on a true relationship with Christ.