The Greatest Commandment

Jesus said, "You shall love the Lord your God with all your heart, with all your soul, and with all your mind." You shall love, not 'fear,' for to love is more than to fear; to fear belongs to slaves, to love to sons; fear is in compulsion, love in freedom. Whoever serves God in fear escapes punishment, but has not the reward of righteousness because he did well unwillingly through fear. God does not desire to be served servilely by men as a master, but to be loved as a father, for that He has given the spirit of adoption to men.

But to love God with the whole heart, is to have the heart inclined to the love of nothing more than God. To love God again with the whole soul is to have the mind stayed upon the truth, and to be firm in the faith. For the love of the heart and the love of the soul are different. The first is in a sort carnal, that we should love God even with our flesh, which we cannot do unless we first depart from the love of the things of this world. The love of the heart is felt in the heart, but the love of the soul is not felt, but is perceived because it consists in a judgment of the soul. For he who believes that all good is in God, and that without Him is no good, he loves God with his whole soul.

Jesus did not only teach the first and greatest commandment, but added that there was a second like to the first, "You shall love your neighbor as yourself." But if Whoever loves sin has hated his own soul, it is shown that he does not love his neighbor as himself, when he does not love himself. But who loves man loves God; for man is God's image, wherein God is loved, as a King is honored in his statue. For this cause the second commandment is said to be like the first. That the second command is like the first signifies that the obligation and merit of both are alike; for no love of God without Christ, or of Christ without God, can lead to salvation. It follows, "On these two commandments hang all the Law and the Prophets."

References:
Opus imperfectum in Mattheum (Pseudo-Chrysostom)
Origen
On the Trinity, Book VIII (St. Augustine)
In Evangelium Matthaei Commentarius (St. Hilary of Poitiers)