1. The purpose of the Old Testament law and prophets was to overcome sin and establish communion between God and his people, as well as among human persons themselves. Thus, love of God and love of neighbor as oneself were indeed the basis of the law and the prophets (see Mt 22.37–40).
2. By the fact of the Incarnation, however, Jesus himself embodies perfect love. Christian life is a sharing in the fullness of love, the personal divine-human communion which Jesus is. Although the law of Moses pointed to this fulfillment, the consummation of the divine-human relationship is reserved to Jesus; the enduring love present in him brings to its climax the love story begun in the Old Testament (see Mt. 5.17; Jn 1.14–17).
3. Not only does Jesus fulfill the law of love, he enables us to fulfill it, too. His love is unique in magnitude and unselfishness, and he commands us to love as he does (see Jn 13.34–35), with a willingness to lay down our lives for our brothers and sisters (see Jn 15.12–14; 1 Jn 3.16). These characteristics of Jesus’ love result from a more fundamental principle: His human love for us is rooted in his divine love, which he receives in being begotten by the Father and which he shares with us. Thus he says: “As the Father has loved me, so have I loved you; abide in my love” (Jn 15.9).
4. Having been made children of God, Christians are to love their heavenly Father and one another as divine children. The requirement to love as Jesus loves us is new, as the new and eternal covenant is new. As God’s sons and daughters, Christians must love as the Son does (see S.t., 1–2. q. 98, aa. 1–2; q. 107, aa. 1–2).
Clearly, the Christian requirement of love is in some way old and in some way new. The First Epistle of John says: “Beloved, I am writing you no new commandment, but an old commandment which you had from the beginning; the old commandment is the word which you have heard. Yet I am writing you a new commandment, which is true in him and in you, because the darkness is passing away and the true light is already shining” (1 Jn 2.7–8). Love always has been a requirement of life in covenant with God, but in the new covenant love is present in a new way.4
Jesus’ love is not merely an example to imitate. Christian love is received and carried out only in the Church, by real unity with Jesus who is the Church’s initiator and head. One loves as Jesus loves only by being united through baptism with his redemptive act, experiencing this unity in the Eucharist, and living it out in one’s daily life. For this reason, Christian love is the unity of the Church, which binds together its many members and harmonizes their diverse gifts (see 1 Cor 12.12–13.13).
In Jesus, God is our neighbor; through him, human persons become—or, at least, are called to become—members of the divine family. Hence Christian love of God includes love of neighbor, and Christian love of neighbor includes love of God. What one does to one’s fellows, one does to the Son of Man, who will separate those to be welcomed into the kingdom from those to be excluded (see Mt 25.31–46).
5. Although primarily a disposition to fulfillment in divine life, Christian love also requires moral goodness, as question B showed. Hence, Christian love disposes both to divine and human goodness—that is, to the perfect accomplishment of the divine-human communion which God is building up upon Jesus (see S.t., 2–2, q. 23, aa. 1, 4). The hope which springs from Christian love will only be satisfied when the fulfillment of all things in Jesus is accomplished, when Jesus hands over the kingdom to the Father, and God is all in all (see 1 Cor 15.20–28; Eph 1.7–10, 22–23; Col 1.18–20).
6. The first principle of all human morality is: In voluntarily acting for human goods and avoiding what is opposed to them, one ought to choose and otherwise will those and only those possibilities whose willing is compatible with a will toward integral human fulfillment (7‑F). Such fulfillment is more than an ideal; it is being accomplished in the fulfillment of all things in Jesus (19‑B). Thus, Christian love transforms the first principle of morality into a more definite norm: One ought to will those and only those possibilities which contribute to the integral human fulfillment being realized in the fulfillment of all things in Jesus.
In the New Testament, one finds several normative statements which are equivalent to the first principle of morality specified in the light of Christian faith.
For example: “Therefore be imitators of God, as beloved children” (Eph 5.1). The idea is that inasmuch as we are children of God, we ought to be like our heavenly Father. Conceptually, this norm is distinct from the first principle of morality specified in the light of Christian faith. However, the author of Ephesians immediately explains how the Christian can put into practice the imitation of God the Father: “And walk in love, as Christ loved us and gave himself up for us, a fragrant offering and sacrifice to God” (Eph 5.2). The sacrificial gift which Jesus made out of love is the act by which the ideal of integral human fulfillment becomes a real possibility, to be realized through the divine power bringing all things to fulfillment in Jesus (see Eph 1.9–10, 22–23; Col 1.15–22). Hence, in fact Christians can imitate God by moral acts only by cooperating with the redemptive love of Jesus and so contributing to fulfillment in Jesus.
Similarly, the norm that Christians should walk according to the Spirit (see Gal 5.16) is in practice equivalent to the normative requirement of Christian love, since the Spirit transforms Christian moral life by communicating divine love. Likewise, the norm of Christian love is not something separate from Jesus in his concrete totality (see LG 42). Love disposes one to that good which will be accomplished in the fulfillment of all things in Jesus.
Again, since what the Church teaches is nothing else than the revealed truth received from Jesus, it follows that an injunction to live according to the Church’ teaching is in practice equivalent (for believing Christians) to the first principle of morality. Hence, no different general principle is proposed when Paul says: “What you have learned and received and heard and seen in me, do” (Phil 4.9).
4. A note in the New American Bible to 1 Jn 2.7–11 provides helpful exegesis: “The law of fraternal charity is based on human nature itself and is confirmed by the divine positive command to the Israelites (Lv 19.18). Through Christ, however, a new and higher relationship with neighbor (verses 8, 10) is achieved as a result of the new relationship with the Father: that of sons of God (cf. Jn 1.12) and brothers of Christ (Lk 8.21).”