Here is how we come to these conclusions, based on Biblical Scripture:
Jesus was asked directly to say what the most important commandment was. His answer came in two parts: Love God more than anything else; love your neighbor as yourself.
MARK 12:28-34 (RSV)
 And one of the scribes came up and heard them disputing with one
another, and seeing that he answered them well, asked him, "Which
commandment is the first of all?"
 Jesus answered, "The first is, 'Hear, O Israel: The Lord our God,
the Lord is one;
 and you shall love the Lord your God with all your heart, and with
all your soul, and with all your mind, and with all your strength.'
 The second is this, 'You shall love your neighbor as yourself.'
There is no other commandment greater than these."
 And the scribe said to him, "You are right, Teacher; you have truly
said that he is one, and there is no other but he;
 and to love him with all the heart, and with all the understanding,
and with all the strength, and to love one's neighbor as oneself, is
much more than all whole burnt offerings and sacrifices."
 And when Jesus saw that he answered wisely, he said to him, "You
are not far from the kingdom of God." And after that no one dared to ask
him any question.
On another occasion, Jesus was asked what was needed to obtain eternal life. His answer here is the very same thing: Love God more than anything else; love your neighbor as yourself.
LUKE 10:25-28 (RSV)
 And behold, a lawyer stood up to put him to the test, saying,
"Teacher, what shall I do to inherit eternal life?"
 He said to him, "What is written in the law? How do you read?"
 And he answered, "You shall love the Lord your God with all your
heart, and with all your soul, and with all your strength, and with all
your mind; and your neighbor as yourself."