From: 1 John 1:5-2:2
God Is Light
 This is the message we have heard from him and proclaim to you, that God is light and in him is no darkness at all.
Walking in the Light. Rejecting Sin
 If we say we have fellowship with him while we walk in darkness, we lie and do not live according to the truth;  but if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin.  If we say we have no sin, we deceive ourselves, and the truth is not in us.  If we confess our sins, he is faithful and just, and will forgive our sins and cleanse us from all unrighteousness.  If we say we have not sinned, we make him a liar, and his word is not in us.
 My little children, I am writing this to you so that you may not sin; but if any one does sin, we have an advocate with the Father, Jesus Christ the righteous;  and he is the expiation for our sins, and not for ours only but also for the sins of the whole world.
1:5-2:29. This section describes what communion with God is, and the demands it makes on us. We can say there are two parts in the section: the first (1:5-2:11) teaches that communion with God means walking in the light and, therefore, rejecting sin and keeping the commandments. The second (2:12-19) warns the readers to guard against worldly concupiscence and not trust false teachers.
St John is writing as a pastor of souls who has lived the life of the Lord and reflected deeply upon it. His teaching interweaves truths of faith with moral and ascetical demands because he wants Christians to live in a way consistent with their faith. Therefore, the text does not really divide into a doctrinal section and a moral section.
5. "God is light": the imagery of light/darkness was much employed in ancient times--sometimes to promote the notion that the world had two principles, one good and the other evil. In St John the image clearly has a different meaning, one connected with biblical teaching on light. When God reveals himself to men, in one way or another light usually plays a part: examples range from the burning bush (cf. Ex 3:1ff) to the coming of the Holy Spirit in the form of tongues of fire (cf. Acts 2:1ff). This imagery is used to show God's sublimity--as we find also in St Paul: "the Lord of Lords,...who dwells in unapproachable light, whom no man has ever seen or can see" (1 Tim 6:15-16).
The image of light also helps to show what revelation involves: God has made himself known to us, enlightening our hearts (cf. 2 Cor 4:6). Thus, we can say that God is light, Jesus Christ has made him known to us, and Christian revelation is the splendor of that light. In St John's Gospel the idea of Christ as the light which enlightens the world occurs very often (cf., e.g., Jn 1:4, 9; 8:12; 9:5). St Thomas Aquinas explains, in this connection, that philosophers prior to Christ had a certain light which allowed them to attain some knowledge of God through reason; the people of Israel had much more light, through divine revelation in the Old Testament; angels and saints, because they have greater knowledge of God by virtue of grace have divine light to a special degree; but only the Word of God is the true light, because he is by his very essence the light which enlightens (cf. "Commentary on St John", 1, 9).
The _expression "God is light" has also a moral dimension: in God there is no darkness because there is no sin; he is sovereign good and all perfection. The light/darkness imagery, therefore, helps to underline the gravity of sin: "the light has come into the world, and men loved darkness rather than light, because their deeds were evil" (Jn 3:19). Those who lead a holy life are called children of light (Jn 12:36; Lk 16:8; Eph 5:8; 1 Thess 5:5); whereas those who do evil live in darkness (1 Thess 5:4), which is the symbol of sin (Lk 22:53).
St John uses the statement that "God is light" to encourage Christians to live in an upright way; as does St Augustine, who comments that we must be united to God and "darkness should be cast away from us so as to allow light to enter, because darkness is incompatible with light" ("In Epist. Joann. ad Parthos", 1, 5).
6-10. The clause "if we say" introduces three suppositions--very probably claims made by some early heretics, especially Gnostics (who boasted of having attained fullness of knowledge and thought they were incapable of sinning).
St John is using the literary technique of parallelism, much employed by Semitic writers: the first sentence states an idea which is repeated and filled out in the later ones. Here, the first statement ("we lie") is later extended to "we deceive ourselves" (v. 8)..., and then to "we make him [God] a liar" (v. 10). This literary device shows that the author of the letter was familiar with this style of writing, very common in the Old Testament.
6-7. Walking in darkness/walking in the light--a graphic description of sinful conduct and upright conduct. St John insists that one cannot justify a life of sin by claiming to have communion with God: "mere confession of faith is in no sense sufficient", St Bede declares, "if that faith is not confirmed by good works" ("In I Epist. S. Ioannis, ad loc.").
"Fellowship with one another": If there were an exact parallelism between the parts of the passage, we would expect it to read "fellowship with him", which is how some Fathers read it. If the text reads differently, it is because mutual communion, the fellowship with the Church to which St John is referring, is a pledge and sign of fellowship with God: "the Church, in Christ, is in the nature of a sacrament--a sign and instrument, that is, of communion with God and of unity among all men" (Vatican II, "Lumen Gentium", 1).
"The blood of his Son Jesus cleanses us from all sin": this idea is often found in the Book of Revelation when it says that the blood of Christ sets us free (cf. Rev 1:5), cleanses souls and makes them white (cf. Rev 7:14), ransoms them for God (cf. Rev 5:9) and defeats the enemies of salvation (cf. Rev 12:11). It is made quite clear that the blood of Christ purifies all types of sin, past and present, mortal and venial. (On the blood of Christ as atonement for all sins, see the notes on Heb 9:12, 14.)
8. "If we say we have no sin": the Old Testament often says that all men are sinners (cf. 7:70; Job 9:2; 14:4; 15:14; 25:4; Prov 20:9; Ps 14:1-4; 51; etc.) and this is also clear from the New Testament (cf. especially Rom 3:10-18). The Council of Trent condemns anyone who says "that a man once justified cannot sin again and cannot lose grace" ("De Iustificatione", can. 23).
Loss of the sense of sin is a danger that threatens man in all epochs. The Apostle's warning (to his contemporaries in the first instance) has particular relevance in our own time." "Deceived by the loss of the sense of sin," John Paul II reminds us, "and at times by an illusion of sinlessness which is not at all Christian, the people of today also need to listen again to St John's admonition, as addressed to each one of them personally: 'If we say we have no sin, we deceive ourselves, and the truth is not in us', and indeed 'the whole world is in the power of the evil one' (1 Jn 5:19). Every individual therefore is invited by the voice of divine truth to examine realistically his or her conscience, and to confess that he or she has been brought forth in iniquity, as we say in the "Miserere" Psalm (cf. Ps 51:7)" ("Reconciliatio Et Paenitentia", 22).
9-10. "If we confess our sins": the Council of Trent quotes this text (without intending to define its exact meaning) when it teaches that confession of sins is of divine institution: 'The Catholic Church has always understood that integral confession of sins was also instituted by the Lord (Jas 5:16; 1 Jn 1:9; Lk 17:14) and is by divine law necessary for all falls after Baptism" ("De Sacramento Paenitentia", chap. 5).
The sacred writer puts emphasis on the interior disposition of the Christian: he should humbly admit that he is a sinner; and St Augustine explains: "If you confess yourself to be a sinner, the truth is in you: the truth is light. Your life does not yet shine as brightly as it might, because there are sins in you; but now you are beginning to be enlightened, because you confess your iniquities" ("In Epist. Joann. Ad Parthos", 1, 6).
"Faithful and just": a translation of two Hebrew words which literally have to do with love and faithfulness. The Old Testament uses this _expression to stress that God's faithful love is always ready to forgive.
1-2. In order to make sure that no one makes a wrong appeal to divine mercy so as to justify their continuing to sin, St John exhorts all to avoid sin. It is one thing to acknowledge that we are sinners and to be conscious of our frailty; it is a very different matter to become completely passive or pessimistic, as if it were not possible to avoid offending God. "Jesus understands our weakness and draws us to himself on an inclined plane," Monsignor Escriva explains. "He wants us to make an effort to climb a little each day. He seeks us out, just as he did the disciples of Emmaus, whom he went out to meet. He sought Thomas, showed himself to him and made him touch with his fingers the open wounds in his hands and side. Jesus Christ is always waiting for us to return to him; he knows our weakness" ("Christ Is Passing By", 75).
"My little children": it is difficult to translate this and other similar expressions in St John, charged as they are with tenderness and a sense of pastoral responsibility. They express a deep, strong love, like that of Jesus at the Last Supper (cf. Jn 13:33). This same Greek term appears six more times in this letter (2:12, 28; 3:7, 18; 4:4; 5: 21); at other times he uses words equivalent to our "my little ones" (cf. 2:14, 18) or "dearly beloved" (2:7; 3:2, 21; 4:1, 7, 11; 3 Jn 2,
5, 11). All these expressions reflect how very close St John was to the faithful.
"We have an advocate with the Father": Jesus Christ, who is the only Mediator (cf. 1 Tim 2:5), intercedes for us. He, who has died for our sins (he is "the expiation"), presents his infinite merits to God the Father, by virtue of which the Father pardons us always. The Holy Spirit is also called Paraclete or Advocate insofar as he accompanies, consoles and guides each Christian, and the whole Church, on its earthly pilgrimage (cf. note on Jn 14:16-17).
"St John the Apostle exhorts us to avoid sin", St Alphonsus says, "but because he is afraid we will lose heart when we remember our past faults, he encourages us to hope for forgiveness provided we are firmly resolved not to fall again; he tells us that we have to put our affairs in order with Christ, who died not only to forgive us but also (after dying) to become our advocate with the heavenly father" ("Reflections on the Passion", Chap. 9, 2).
Source: "The Navarre Bible: Text and Commentaries".