Albert Barnes Commentary Mark 12:29

"Jesus answered, The first is, Hear, O Israel; The Lord our God, the Lord is one:" — Mark 12:29 (ASV)

Hear, O Israel. This was said to call their attention to the great importance of the truth about to be proclaimed. See Deuteronomy 6:4, 6.

The Lord our God, etc. Literally, "Jehovah, our God, is one Jehovah." The other nations worshipped many gods, but the God of the Jews was one, and one only. Jehovah was undivided; and it was the design of the separation of the Jewish people from other nations to keep this great truth in mind. This was the distinctive truth which was communicated to the Jews, and this they were required to keep and remember forever.