C. F. Keil and Franz Delitzsch on Exodus 19:7-9

Exo 19:7-9

When Moses communicated to the people through their elders this incomparable promise of the Lord, they promised unanimously (יחדּו) to do all that Jehovah said; and when Moses reported to the Lord what the people had answered, He said to Moses, “I will come to thee in the darkness of the cloud, that the people may listen to My speaking to thee (בּ שׁמע, as in Gen 27:5, etc.), and also believe thee for ever.” As God knew the weakness of the sinful nation, and could not, as the Holy One, come into direct intercourse with it on account of its unholiness, but was about to conclude the covenant with it through the mediation of Moses, it was necessary, in order to accomplish the design of God, that the chosen mediator should receive special credentials; and these were to consists in the fact that Jehovah spoke to Moses in the sight and hearing of the people, that is to say, that He solemnly proclaimed the fundamental law of the covenant in the presence of the whole nation (Ex 19:16-20:18), and showed by this fact that Moses was the recipient and mediator of the revelation of God, in order that the people might believe him “for ever,” as the law was to possess everlasting validity (Mat 5:18).