Jamieson, Fausset and Brown on Exodus 19:3-6

3-6. Moses went up unto God--the Shekinah--within the cloud (Ex 33:20; Joh 1:18).

Thus shalt thou say to the house of Jacob, &c.--The object for which Moses went up was to receive and convey to the people the message contained in these verses, and the purport of which was a general announcement of the terms on which God was to take the Israelites into a close and peculiar relation to Himself. In thus negotiating between God and His people, the highest post of duty which any mortal man was ever called to occupy, Moses was still but a servant. The only Mediator is Jesus Christ [1Ti 2:5; He 12:24].