1. II. Light and Darkness (1:5-2:24)
    1. C. Test of Love (2:7-2:17)
      1. i. Call to love one another (2:7-2:11)
Thematic Relation: The commandment has always been to love your God and your brother.

Some Key Words

True (alethes[227]):
true. This is used (2:8) of the commandment.
True (alethinos[228]):
genuine, real; having not only the name, but the real nature, and in that respect perfect. This is used (2:8) of the light that now shines.
Hate(miseo [3404]):
to detest, hate; to pursue with hatred.
Brother(adelphos[80]):
a fellow believer, united by the bond of affection. In its broadest sense, any man. Oof.
Love(agapao[25]):
a direction of the will; finding one's joy in a thing; to befriend; to be fond of.
Stumbling(skandalon[4625]):
occasion to fall; a thing that offends; the trigger of a trap; anything by which one is drawn into sin.
 

Paraphrase

7 The rule of life laid down in this letter is both old - going back to the Mosaic law 8 and new - inasmuch as we have been empowered of God to live by it. 9-11 Just as light and darkness cannot mix, so neither can claims of being God's children mix with displays of hatred.

Key Verse

2:10 - the passing of the test consists of loving your brother, which gives no cause for stumbling.

Thematic Relevance:

 Love is the mark of the believer. It has ever been God's command to love Himself and to love our neighbors. Those who love Him, love His commands and keep them. Thus, our love for others becomes a proof of our love of Him.

Doctrinal Relevance:

 There is a counter to the Antinomian position here, in that the Mosaic law is shown to be still in effect - if anything, strengthened by our newly given ability to live by it.

Moral Relevance:

 Again, our actions are required to match our claims. We cannot love God without loving those He loves.

Some Parallel Verses

2:7
Jn 13:34 - Christ declares a 'new' commandment, which in reality is an amplification of the old. 2Jn 5 - which John asks his fellow believers to follow. 2Jn 6 - which love is to walk according to His commandments. 
2:8
Ro 13:12 - we are to put aside our old ways of sin, and walk in purity. Eph 5:8 - which will show in goodness, righteousness, and truth. 1Th 5:4 - we are not of darkness. Jn 1:9 - Christ is the true light.
2:9
2:10
Jn 11:9 - those who walk by day don't stumble, because they can see.
2:11
Jn 12:35 - those who walk in darkness don't know where they go. 2Co 4:4 - because they have been blinded by Satan, so as not to see the light revealed in Christ. 2Pe 1:9 - Those who don't walk according to their confession have forgotten their purification, and gone back to a life of blindness.

New Thoughts

If we are to take “brother” in its broadest sense – which, I think, is what Jesus’ example would show to be correct – then the test of 2:10 is even harder than one might already think. We are to have a genuine fondness for our fellow man, to show the complete and perfect truth of love for them, finding our joy in them (not in their sin, though). God, Your ways are hard to follow. The path You place us upon is challenging to the extreme. I pray You would help us to lay hold of the strength You provide to make us able. Lord, help us to will to do Your will, to live the life You require of us. And Lord, provide for us the ability to do all that You would have us to do.