What is the most important rule of life?
The answer is found in the gospels - this from Matthews'.
Hearing that Jesus had silenced the Sadducees, the Pharisees got together. One of them, an expert in the law, tested him with this question: “Teacher, which is the greatest commandment in the Law?” Jesus replied: “ ‘Love the Lord your God with all your heart and with all your soul and with all your mind.’ This is the first and greatest commandment. And the second is like it: ‘Love your neighbour as yourself.’ All the Law and the Prophets hang on these two commandments.”
So Jesus taught that the most important thing is to love God and the second is to love others.