This basic command to love originates in the Old Testament. With all thy heart ... soul ... mind ... strength. The interpreter should resist the temptation to find subtle differences between these words. They are used here to show that love must flow from the entire person. Love cannot be separated into compartments. The suggestion is that we will either love God with our all, or not at all. This is the first commandment. Jesus did not mean that this is the first commandment given in time, but that it is the first commandment in terms of priority.
Source: Biblical Family Values: Adult Teacher's Insights, p. 5.