But that the covenant of works was also, for special ends, repeated and delivered to the Israelites on Mount Sinai, I cannot refuse—

1. Because of the apostle’s testimony, ‘These are the two covenants: the one from Mount Sinai which gendereth to bondage’ (Gal. 4:24). For the children of this Sinai covenant the apostle here treats of, are excluded from the eternal inheritance as Ishamel was from Canaan, the type of it, ‘Cast out the bond-woman and her son; for the son of the bond-woman shall not be hero with the son of the free woman’ (v. 30); but this could never be said of the children of the covenant of grace under any dispensation, though both the law and the covenant from Sinai itself, and its children, were even before the coming of Christ under a sentence of exclusion, to be executed on them respectively in due time.

2. The nature of the covenant of works is most expressly in the New Testament brought in, propounded, and explained from the Mosaical dispensation. The commands of it from Exodus 20 by our blessed Savior, ‘If thou wilt enter into life keep the commandments. He said unto him, Which? Jesus said, Thou shalt do no murder, thou shalt not commit adultery…’ (Matt. 19:17–19). The promise of it, ‘Moses describes the righteousness which is of the law, that the man which doth these things shall live by them’ (Rom. 10:5). The commands and promise of it together, see Luke 10:25–8. The terrible sanction of it, Galatians 3:10. For it is written (viz: Deuteronomy 27:26), ‘Cursed is everyone that continueth not in al things which are written in the book of the law to do them.’

3. To this may be added the opposition betwixt the law and grace, so frequently inculcated in the New Testament, especially in Paul’s epistles…..

4. The law from Mount Sinai was a covenant, ‘These are the two covenants, the one from the Mount Sinai’ (Gal. 4:24); and such a covenant as had a semblance of disannulling the covenant of grace, ‘The covenant that was confirmed before of God in Christ, the law which was 430 years after, cannot disannul’ (Gal. 3:17); yea, such an one as did, in its own nature, bear a method of obtaining the inheritance, so far different from that of the promise, that it was inconsistent with it; ‘For if the inheritance be of the law, it is no more of promise (Gal. 3:18), wherefore the covenant of the law from Mount Sinai could not be the covenant of grace unless one will make this last not only a covenant seeming to destroy itself, but really inconsistent: but it was the covenant of works, which indeed had such a semblance, and in its own nature did bear such a method as before noted….

—Thomas Boston’s Notes on The Marrow of Modern Divinity, 76–77.