Albert Barnes on Matthew 22:36

Which is the great commandment? That is, the greatest commandment, or the one most important. The Jews are said to have divided the law into greater and smaller, commandments. Which was of the greatest importance they had not determined. Some held that it was the law respecting sacrifice; others, that respecting circumcision; others, that pertaining to washings and purifying, etc.

The law. The word law has a great variety of significations; it means, commonly, in the Bible, as it does here, the law given by Moses, recorded in the first five books of the Bible.