The commandment to love our neighbor is similar to that by which we are bound to love God, since it is for love of Him that we love our neighbor at all.
“Two-fold,” says St. Augustine, “is the commandment but one is the charity.”
And to enforce His teaching in the eyes of the Pharisees, our Lord gives them a proof of His Divinity in the shape of a passage quoted from David himself. We ought, therefore, to be united to Christ by faith and love. Questioned about the first commandment, our Lord reveals the second, which is scarcely of less importance than the first, thus showing them that they only questioned Him through hatred since charity does not ask perverse questions (I Cor xiii, 4). In this way He shows His respect for the law and the prophets.
Having thus given His answer Jesus asks questions in His turn and proves that since being the Son of David, He is called his Lord, He is the only Son of the Father. And He alarms them by telling them that one day He will triumph over all who resist His rule, for God will make His enemies His footstool. By these words He makes clear the union and harmony existing between His Father and Himself. The Gradual psalm sings to us of the Divine charity: “Blessed is the people whom He hath chosen for His inheritance. Let Thy mercy, O Lord, be upon us, as we have hoped in Thee.”
All men are summoned to the Kingdom of God, the heavenly Jerusalem animated by charity, to form “one body” that is the Church, called by St. Gregory a new creation, and quickened by “one Spirit”, the Holy Ghost given at Pentecost, for there exists for all but “one Lord, one faith, one baptism” (Epistle).
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.