The passage
      v1-3. The opening remark, "How great", is an expression of amazement. God has acted with self-giving love to bring us into an intimate relationship with himself; he has made us his children. We do not deserve his love and so this makes his self-givine toward us even more wonderful. Three consequences flow from this truth:
        i] "The world does not know us." The world does not know us in the same sense as it does not know God. The child of God is not recognized by secular society; we are ignored, even condemned.
        ii] "What we will be has not yet been made known". Our standing in Christ enables us to deal with the present limitations that we must endure, cf. Deut.29:29. So, we live with our limited understanding, knowing that one day we will stand before the Father and see him as he is, and as he is, so shall we be. In that day we will know even as we are known.
        iii] "Everyone who has this hope, purifies themselves." Holiness, or Christ likeness, is our hope, it is our destiny. In fact, as far as God is concerned, we are already perfect. So, we strive to purify ourselves, strive to be what we are. Our destiny is purity, perfection, so we strive to be pure. This is a natural response to our being in a relationship with the divine. "Everyone who has this hope" seeks to live in purity. God "is pure" and so it is only natural that His children should push themselves toward purity. It is often suggested that the example of Christ's life is what motivates us in this quest for perfection. His goodness, his purity, drives us toward purity. Yet, it is more likely that the motivating force is the same motivating power that stood with Christ in his life's journey. Only the Holy Spirit can sanctify and we need only cooperate with his work of renewal for us to be what we are.
      v4-6. John's argument now runs as follows: Everyone who sins finds themselves in rebellion against God, lawless. Jesus came to deal with this state of loss. He, the perfect Son of God, took upon himself the punishment for our state of rebellion, Jn.1:29, 10:15. Thus, as a consequence, those who are in a relationship with Jesus press toward perfection rather than corruption.
      In this passage John identifies two Christian groups, one genuine and the other fraudulent. The "fruit" of their lives evidences whether they are in fellowship with Christ, or not. You can't be in fellowship with a sinless saviour and continue to push toward corruption. John's words seem to imply, on the surface at least, that a disciple of Christ should be sinless. Yet, this is clearly not what he is saying. In 1:8 and 10 John lays the groundwork for his argument. At the beginning of his letter he makes the point that we cannot claim to be without sin. So, he is not speaking about a realized perfection, for "sinning", "lawlessness", remain part of our daily experience. John is concerned about orientation, not personal purity as such. The issue here is that believers are assured of their standing in Christ because of their natural orientation toward the perfection they already possess in Christ.

In the family of God
      One of the greatest problems facing believers these days is a lack of assurance. Scratch a believer and you will find someone unsure of their salvation. The reason for this lack of assurance is often related to our clumsy attempts to confirm our salvation by means of our personal piety. We work at proving our standing with God on the basis of our goodness, but every day we fail miserably and so find our standing undermined.
      Take note of John's point. God's love is beyond calculation, for not only does he call us his children, he makes us his children. The day is coming when we, the children of God, will not only know him, but we will be like him. No matter how great are our weaknesses or failings, they can't get in the way of God's gracious love for us. Jesus has taken away our sins and we are his forever.
      This truth produces great confidence, it produces great assurance, and this assurance is supported by an amazing fact: a believer is orientated toward righteousness. This doesn't mean that a believer won't sin, in fact, sin will daily infect our lives. What it means is that a believer is attracted toward Christ-likeness. A person who has truly experienced the mercy of God will tend to be merciful - not perfectly merciful, but oriented toward mercy. A forgiven person forgives, strives to forgive. A person bathed in the purity of Jesus tries to express that purity in their lives.
      So, our standing as God's children rests on his grace alone. As for our personal piety, it but demonstrates orientation, an orientation toward Christ, or an orientation toward lawlessness. As for sin, the truth is there is no sinless Christian; we all trip and fall.

v2
      oupw efanerwqh (fanerow) aor. pas. "not yet been made known" - it was not yet revealed, manifested. "What we shall be has not yet been revealed", Barclay.
      ean + subj. "when" - if. Although grammatically introducing a conditional sentence 3rd class, the strength of "we know that" prompts the relative pronoun + ean + subj. to form "that if" which is translated as "when / whenever"
      fanerwqh/ (fanerow) aor. pas. subj. "[when] he appears" - he is manifested. The translation depends on the subject. The NIV takes the subject as Jesus/God, but it could well be "what we will be", in which case the translation would be "when it does become evident."
      oJmoioi adj. "[we shall be] like [him]" - like. Reflecting the glory of the Creator. "We only know that if reality were to break through, we should reflect his likeness", Phillips.
      autw/ "[like] him" - Like Jesus? Again, the context implies God, therefore we will take on divinity.
      oyomeqa (oJraw) fut. "we shall see" - we will see.

v3
      elpida (iV, idoV) "hope" - A confident expectation in the promises of God. Note, the hope is "on him", not on some imagined intention or desire.
      aJgnizei (aJgnizw) pres. "purifies" - purifies. The word is used for both devotional purity and ethical purity. "This hope makes us keep ourselves holy", CEV.
      ekeinoV "he" - that one. Referring to God. Bultmann argues that "he" refers to Jesus, while on the other hand, commentators like Pink argue it refers to God.

v4
      oJ poiwn pres. part. "[everyone] who [sins]" - [all] the ones doing [the sin]. Participle as a substantive. "Everyone who commits sin", NRSV; "to commit sin is to break God's law", Barclay.
      poiei (poiew) "breaks [the law]" - does [lawlessness]. "Is guilty of lawlessness", NRSV.
      kai hJ aJmartia estin hJ anomia "in fact, sin is lawlessness" - and sin is lawlessness. "In fact" is rightly "and", although some suggest "for."

v5
      oidate (oida) perf. "you know" - know. Obviously, know by means of communication, from both divine revelation and Christian teachers. The knowledge of Christ's atonement is not something that can be worked out through human reasoning.
      oJti "that" - because, since, that. Most often "because/for", but when introducing a noun clause, dependent statement of perception, "that". Here defining the content of the knowledge.
      ekeinoV "he" - the one. Obviously Christ is intended here.
      arh/ (airw) aor. subj. "take away" - he might bear, carry, remove, take away, lift up. "Christ appeared to remove sin altogether", Barclay.
      en autw/ "in him" - Again Christ is obviously intended here.
      estin "[in him] is [no sin]" - Note, the tense is present. Christ is alive and without sin continually "He isn't sinful", CEV.