Sinai Covenant Added, Driving Israelites to Trust in the Promise/Covenant of Grace

"Hence we may conclude that the end which God aimed at in giving the law to Moses was not that any should ever attain to holiness or salvation by the condition of perfect or sincere obedience to it, though, if there had been any such way of salvation at that time, it must have consisted in the performance of that law, which was then given to the church to be a rule of life, as well as a covenant. There was another covenant made before that time with Abraham, Isaac and Jacob, a covenant of grace, promising all blessings freely through Christ, the promised seed, by which only they were to be saved. And the covenant of the law was added that they might see their sinfulness and subjection to death and wrath, and the impossibility of attaining to life or holiness by their works, and be forced to trust on the free promise only for all their salvation, and that sin might be restrained by the spirit of bondage until the coming of that promised seed Jesus Christ, and the more plentiful pouring out of the sanctifying Spirit, by Him. This the apostle Paul shows largely (Gal. 3: 15 -24; Rom. 5: 20, 21; 10: 3, 4). None of the Israelites under the Old Testament were ever saved by the Sinai covenant; neither did any of them ever attain to holiness by the terms of it. Some of them did indeed perform the commandments of it sincerely, though imperfectly, but those were first justified, and made partakers of life and holiness, by virtue of that better covenant made with Abraham, Isaac and Jacob, which was the same in substance with the new covenant or testament established by the blood of Christ. Had it not been for that better covenant, the Sinai covenant would have proved to them an occasion of no happiness, but only of sin, despair and destruction. Of itself it was only a killing letter, the ministration of death and condemnation, and therefore it is now abolished (2 Cor. 3: 6, 8, 9, 11). 

"We have cause to praise God for delivering His church, by the blood of Christ, from this yoke of bondage; and we have cause to abhor the device of those that would lay upon us a more grievous and terrible yoke, by turning our very new covenant into a covenant of sincere works, and leaving us no such better covenant, as the Israelites had under their yoke, to relieve us in our extremity."

Walter Marshall. The Gospel Mystery of Sanctification, pp 100-101