From: Mark 12:28-34
The Greatest Commandment of All
 One of the scribes came up and heard them disputing with one another, and seeing that He (Jesus) 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 than He;  and to love 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.
28-34. The doctor of the law who asks Jesus this question is obviously an upright man who is sincerely seeking the truth. He was impressed by Jesus' earlier reply (verses 18-27) and he wants to learn more from Him. His question is to the point and Jesus devotes time to instructing him, though he will soon castigate the scribes, of whom this man is one (cf. Mark 12:38ff).
Jesus sees in this man not just a scribe but a person who is looking for the truth. And His teaching finds its way into the man's heart. The scribe repeats what Jesus says, savoring it, and our Lord offers him an affectionate word which encourages his definitive conversion: "You are not far from the Kingdom of God." This encounter reminds us of His meeting with Nicodemus (cf. John 3:1ff). On the doctrinal content of these two commandments cf. note on Matthew 22:34-40.
[Note on Matthew 22:34-40 states:
In reply to the question, our Lord points out that the whole law can be condensed into two commandments: the first and more important consists in unconditional love of God; the second is a consequence and result of the first, because when man is loved, St. Thomas says, God is loved, for man is the image of God (cf. "Commentary on St. Matthew", 22:4).
A person who genuinely loves God also loves his fellows because he realizes that they are his brothers and sisters, children of the same Father, redeemed by the same blood of our Lord Jesus Christ: "This commandment we have from Him, that he who loves God should love his brother also" (1 John 4:21). However, if we love man for man's sake without reference to God, this love will become an obstacle in the way of keeping the first commandment, and then it is no longer genuine love of our neighbor. But love of our neighbor for God's sake is clear proof that we love God: "If anyone says, `I love God', and hates his brother, he is a liar" (1 John 4:20).
"You shall love your neighbor as yourself": here our Lord establishes as the guideline for our love of neighbor the love each of us has for himself; both love of others and love of self are based on love of God. Hence, in some cases it can happen that God requires us to put our neighbor's need before our own; in others, not: it depends on what value, in light of God's love, needs to be put on the spiritual and material factors involved.
Obviously spiritual goods take absolute precedence over material ones, even over life itself. Therefore, spiritual goods, be they our own or our neighbor's, must be the first to be safeguarded. If the spiritual good in question is the supreme one for the salvation of the soul, no one is justified in putting his own soul into certain danger of being condemned in order to save another, because given human freedom we can never be absolutely sure what personal choice another person may make: this is the situation in the parable (cf. Matthew 25:1-13), where the wise virgins refuse to give oil to the foolish ones; similarly St. Paul says that he would wish himself to be rejected if that could save his brothers (cf. Romans 9:3)--an unreal theoretical situation. However, what is quite clear is that we have to do all we can to save our brothers, conscious that, if someone helps to bring a sinner back to the way, he will save himself from eternal death and cover a multitude of his own sins (James 5:20). From all this we can deduce that self-love of the right kind, based on God's love for man, necessarily involves forgetting oneself in order to love God and our neighbor for God.]
30. This commandment of the Old Law, ratified by Jesus, shows, above all, God's great desire to engage in intimate conversation with man: "would it not have sufficed to publish a permission giving us leave to love Him? [...]. He makes a stronger declaration of His passionate love for us, and commands us to love Him with all our power, lest the consideration of His majesty and our misery, which make so great a distance and inequality between us, or some other pretext, divert us from His love. In this He well shows that He did not leave in us for nothing the natural inclination to love Him, for to the end that it may not be idle, He urges us by His general commandment to employ it, and that this commandment may be effected, there is no living man He has not furnished him abundantly with all means requisite thereto" (St. Francis de Sales, "Treatise on the Love of God", Book 2, Chapter 8).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.