First, John says: God is love, and those who abide in love abide in God, and God abides in them (v. 16). This constitutes a straightforward statement of the nature of John's understanding of deification. Because the nature of God is love (cf. also v. 8), therefore those who abide in God -- who live in the life of the Holy Trinity, through the enlightenment of the Holy Spirit as adopted sons and co-heirs with Jesus Christ -- must be characterized by love. What God is by nature, Christians become through participation by grace. Importantly, too, there is an element of voluntary participation and collaboration with God: those who would abide in God must abide, must remain, must stay in the life of love. Certainly this love is not produced by them -- it comes from God (v. 7) -- but we must cooperate with God in this process.
Second, John says: Love has been perfected among us in this: that we may have boldness on the day of judgment, because as he is, so are we in this world (v. 17). Notice the boldness to which John appeals: our boldness on the day of judgment comes from the fact that, through God's grace and through our own cooperation with him, we have become like him. When we will see God -- perhaps this is not too radical to say! -- we will see something of ourselves, because we have been transformed into his image and likeness through his mercy. Maybe it will be better to say that God will see himself in us. In any case, the boldness on the day of judgment comes from that which, by God's help, we have become. We will find ourselves at home and at rest in the presence of God: he won't be a stranger but someone rather familiar.
An important corollary of this is that, just as I have emphasized previously, the work of the Holy Spirit is to reconcile us to God. Our relationship with God is elevated to one of friendship, rather than servile fear: I do not call you servants any longer, because the servant does not know what the master is doing; but I have called you friends, because I have made known to you everything that I have heard from my Father (John 15:15). Cast away is every fear or punishment or of rejection; in its place is planted the seed of righteousness: a genuine and true love for God. Thus John repeats: There is no fear in love, but perfect love casts out fear; for fear has to do with punishment, and whoever fears has not reached perfection in love (v. 18).
Importantly, too, the deified person recognizes that this is all due to the grace of God: We love because he first loved us (v. 19). There is no pride in all of this, nor is there any room for boasting, though we do not negate the necessity of the human person's faithfulness and persistence in God's grace through her own effort. As St. Mark the Ascetic wrote, if we are righteous, we are only giving God what is due to him in the first place; but he gives us eternal life and the gift of deification as a gift in response to our efforts:
A slave does not demand his freedom as a reward; but he gives satisfaction as one who is in debt, and he receives freedom as a gift. (§3)
A master is under no obligation to reward his slaves; on the other hand, those who do not serve him well are not given their freedom. (§19)
When Scripture says 'He will reward every man according to his works' (Matt. 16: 27), do not imagine that works in themselves merit either hell or the kingdom. On the contrary, Christ rewards each man according to whether his works are done with faith or without faith in Himself; and He is not a dealer bound by contract, but God our Creator and Redeemer. (§22; On Those who Think that They are Made Righteous by Works, in Philokalia, vol. I)So we must think of ourselves in this way: Do you thank the slave for doing what was commanded? So you also, when you have done all that you were ordered to do, say, ‘We are worthless slaves; we have done only what we ought to have done!’ (Luke 17:9-10).
John goes on to emphasize the inextricably social aspects to deification: Those who say, "I love God," and hate their brothers or sisters, are liars; for those who do not love a brother or sister whom they have seen, cannot love God whom they have not seen (v. 20). As I have emphasized in a previous post, if God is love and this means he loves all people and works towards their salvation, this means further that he wishes to do it through you and me. This is the high calling and grace of God towards humans: the offer that they be the instruments through which he saves this world. But if a person instead adopts a stance of indifference or even hatred towards another person, that is tantamount to turning one's back on God, as well, for God cannot be separated from the person about whom he is concerned. We ought to check ourselves often, to see if indifference or hatred for a bother is separating us from God, who is love.
And so the final verse: The commandment we have from him is this: those who love God must also love their brothers and sisters (v. 21). There is much to say here, but I want to emphasize one thing only. The deified person, I think, does not see the commandments of God as burdensome or tedious, but rather takes joy in them. He sees that they are right, he rejoices in them, he finds they resonate with what he feels on the inside and what he knows. Thus John later says: For the love of God is this, that we obey his commandments. And his commandments are not burdensome (5:3). Those persons who teach about a God of "grace" who does away with all commandments, or who does not impose any commandment or necessity on the Christian, are coming very close to teaching a false god. Christ himself said: You are my friends if you do what I command you (John 15:14). If your Christ does not command us anything, does not expect anything of us, then he is not John's Christ, and certainly not the Christ of the apostles.