God also spoke to Moses and said to him: "I am the Lord. I appeared to Abraham, Isaac, and Jacob as Kel Shakkai, but by my name YKVK I did not make myself known to them.Literary biblical critics find in this passage evidence of multiple sources of the Pentateuch. After all, the name YKVK is found prior to this statement. It must have been, many argue, that in this particular source document the name YKVK was not used earlier. In other source documents, however, the name was.
(note that intentional minor changes were made to God's name in case some readers feel the need to say them out loud)
Dr. Meir Seidler of Bar Ilan University introduces readers to the non-traditional Jewish commentator Benno Jacob (1862-1945) and offers his philological defense of Rashi's (by definition, traditional) interpretation of this passage that removes any implication of multiple source documents.
The key, according to Rashi and Jacob, is the phrase "make myself known." This does not mean that God was not known by this name but, rather, that He did not act as YKVK. See here for more elaboration.