“What.” The Pharisee asks this question in a respectful manner, and it was an honest question, and designed to “test” Jesus. Jesus had just silenced the Sadducees on the topic of resurrection, which delighted the Pharisees. The Sadducees and Pharisees also were sharply divided over which commandments were important and which were not. The Sadducees asserted that a commandment had to be in the Torah, the first 5 books of Moses, while the Pharisees had a much broader interpretation. This Pharisee wanted to see how Jesus would answer, and whether it would support a Pharisaical position or not.
“first.” The Greek word translated “first” is prōtos (#4413 πρῶτος), and it can mean first in time, first in place, first in rank, honor or power. Here it means first in rank. We would normally say, “most important.” “What is the most important commandment in the Law?” The answer is important, because it turns out that the most important commandment was not even one of the Ten Commandments, although it is certainly implied because if we have no other gods before God, and if we obey the Ten Commandments, then we clearly love God. Nevertheless, the statement that we love God with all our heart, soul, mind, and strength, was an amplification and a clarification of the rest of the Law.