16 The test of true love is identified as willingness to sacrifice one's life for one's brother. The demonstrative "this" that begins the statement points backward to the negative example of Cain and forward to the positive example of Christ. Love is used absolutely and its reference point is Christ's death. The demand for love thus arises from his command, and the meaning of love is found in his example.
"We know" suggests that the knowledge that is involved belongs to events of Jesus Christ, which they heard from the beginning (cf. 1:1; 3:11). "Love" (agape) cannot be derived from some intuitive grasp of an idea but is known in the historical event in which Jesus Christ laid down his own life for us (cf. Jn 10:11). His sacrificial death thus distinguishes agape love from all other loves by its costliness, its unconditional acceptance of another, and its accomplishment.
The personal commitment of Christ is expressed in the words of Jn 15:12-13 (cf. 13:1): "Love each other as I have loved you. Greater love has no one than this, that one lay down his life for his friends." Its accomplishment as a "for us" kind of love is reflected in Jesus' work. It is clear that Jesus understood his death as an effectual, accomplishing act for giving us eternal life (Jn 10:28); it was the only method open to him to fulfill his Father's will (cf. Jn 10:11-18, 27-30; 15:9-18; 17:19). Since agape love is grounded in Jesus death for us, knowledge of it can be received only where his "death" is appropriated into our experience.
The dramatic conclusion we are irresistibly led to is this: "And we ought to lay down our lives for our brothers." We are to do this not simply because that is what Jesus did, but because that is what Jesus revealed to be the demand of agape love. Love is denial of self for another's gain.
17 Again John's penchant for providing practical "tests" of the validity of one's faith comes to the fore. How can we know whether we would sacrifice our life for a fellow believer? We can know by being compassionate toward such a one in his or her present need. If we are unable or unwilling to sacrifice material advantage for the sake of our fellow believers, we know that the love of God is not in us. What are the conditions for our involvement? If we are in a position to see with our own eyes someone's need (as, for example, the good Samaritan did, Lk 10:29-37) and can offer help, then we cannot do otherwise than act. To withhold help in such a situation is to shut off compassionate action and to deny the presence of God's love in our own heart.
18 Turning back to "little children", John admonishes with the tone of a spiritual father pleading for the heartfelt response of his children. Love requires more than idle talk or exalted theology. It demands simple acts, which anyone can see, in order to meet the needs of brothers and sisters in distress. Any expression of love that fails here is not only empty but blasphemous (cf. Jam 2:15-16; 1 Cor 13:1-3).
[NIV BIBLE COMMENTARY Volume 2: New Testament].
JESUS IS LORD.