The clause may be paraphrased, “If, on the other hand, instead of walking in darkness, we walk ...” John does not leave his flock in despair but turns to the positive aspects of the Christian life, thereby to encourage them and express his confidence in them.

 God is constantly surrounded by light that radiates from Himself. The best that Christians can do is to walk in the rays of light that emanate from God. As a traveler will follow the light of a guide along a dark and unknown road, so the child of God will follow light from the Lord along the road of life (2 Cor. 4:6; Eph. 5:8; cf. on Prov. 4:18).

 If we walk in the light we walk with God, from whom the light shines, and have fellowship not only with Him but also with all others who are following the Lord. Serving the same God, believing the same truths, following the same instructions on the pathway of life, we cannot fail to walk in unity. The slightest sign of ill will between us and our brethren should make us review our own conduct, to be sure that we are not veering away from the lighted path of life (cf. on ch. 4:20).

 The last clause of the verse is by no means an afterthought, for the experience here described is closely connected with walking “in the light.” Recognizing that even those who fellowship with God will continue to need cleansing from sin, John assures the Christian that God has already anticipated this need and provided for it. For the significance of “blood” in cleansing from sin see on Rom. 3:25; 5:9; cf. on John 6:53.

 Important textual evidence may be cited (cf. p. 10) for the omission of “Christ.” But because in his epistles John often uses the expression “Jesus Christ” or speaks of Jesus as “the Christ” or “the Son of God” (chs. 4:15; 5:1, 5), many prefer to retain the word Christ. In his Gospel the apostle often speaks of Jesus, the incarnate Word, but here he is thinking particularly of the divine-human Saviour, Jesus Christ. For the title Jesus Christ see on Matt. 1:1.

 This further identification of Jesus emphasizes the magnitude of the sacrifice that provided the cleansing blood—it came from the Son of God. For a discussion of the Sonship of Christ see on Luke 1:35.

 Gr. katharizō, “to make clean,” “to cleanse,” used in the Gospels for “cleansing” a leper (Matt. 8:2; Luke 4:27; etc.), and elsewhere for cleansing from sin or from the guilt of sin (2 Cor. 7:1; Eph. 5:26; Heb. 9:14; etc.). The cleansing to which John here refers is not that which occurs with first repentance and confession, at the beginning of the Christian walk, and which precedes the fellowship. The cleansing here spoken of continues throughout the earthly life, and is part of the process of sanctification (see on Rom. 6:19; 1 Thess. 4:3). None but Christ has ever lived a sinless life (see on John 8:46; 1 Peter 2:22); so men continually need Christ’s blood to cleanse them from their sins (see on 1 John 2:1, 2).

 The writer includes himself among those who need this cleansing. Those who walk nearest to God will, in the glory of His light, be most conscious of their own sinfulness (ch. 1:8, 10; AA 561, 562; GC 469-473).

 Rather, “every sin,” that is, every type and manifestation of sin. For comment on “sin” see on ch. 3:4.