the LORD said. See note on Exodus 3:7, and p. note on Exodus 6:10.
Lo. Figure of speech Asterismos. App-6.
speak... believe. Compare Romans 10:17. "Faith cometh by hearing".
Related Commentaries of Exodus 19:9
Exodus 19:9
9 And the LORD said unto Moses, Lo, I come unto thee in a thick cloud, that the people may hear when I speak with thee, and believe thee for ever. And Moses told the words of the people unto the LORD.