Jamieson, Fausset and Brown on Exodus 34:6

6. the Lord passed by before him--in this remarkable scene, God performed what He had promised to Moses the day before.

proclaimed, The Lord ... merciful and gracious--At an earlier period He had announced Himself to Moses, in the glory of His self-existent and eternal majesty, as "I am" [Ex 3:14]; now He makes Himself known in the glory of His grace and goodness--attributes that were to be illustriously displayed in the future history and experience of the church. Being about to republish His law--the sin of the Israelites being forgiven and the deed of pardon about to be signed and sealed by renewing the terms of the former covenant--it was the most fitting time to proclaim the extent of the divine mercy which was to be displayed, not in the case of Israel only, but of all who offend.