Which is the greatest commandment (Matt. 22:34-40; Mark 12:28-34) is the third of the four great questions the leaders of Israel asked Yeshua, scrutinizing the chosen Lamb of God leading up to one great Passover. The pinnacle of all these discussions is about the purpose and goal of the Law.
There isn’t a directly parallel “greatest commandment” passage in Luke, but there were earlier passages (Luke 10:25-37; 18:18-27) in which Yeshua was questioned about which commandments are connected to “eternal life.”
A distillation of the Law is meant to focus one’s attention on the “weightier matters of the Law,” rather than replace even the seemingly insignificant ones.