2:3 This is how we know that we know him: if we keep his commandments.
2:4 One who says, "I know him," and doesn't keep his commandments,
is a liar, and the truth isn't in him.
2:5 But whoever keeps his word,
God's love has most assuredly been perfected in him.

This is how we know that we are in him:
2:6 he who says he remains in him ought himself also to walk just like he walked.

2:7 Brothers, I write no new commandment to you,
but an old commandment which you had from the beginning.
The old commandment is the word which you heard from the beginning.
2:8 Again, I write a new commandment to you, which is true in him and in you;
because the darkness is passing away, and the true light already shines.

2:9 He who says he is in the light and hates his brother,
is in the darkness even until now.
2:10 He who loves his brother remains in the light,
and there is no occasion for stumbling in him.
2:11 But he who hates his brother is in the darkness, and walks in the darkness,
and doesn't know where he is going, because the darkness has blinded his eyes.

2:15 Don't love the world, neither the things that are in the world.
If anyone loves the world, the Father's love isn't in him.
 

2:16 For all that is in the world,
the lust of the flesh, the lust of the eyes,
and the pride of life, isn't the Father's, but is the world's.

2:17 The world is passing away with its lusts,
but he who does God's will remains forever.

In what practical ways have you been showing love to other Christians?
Do such acts dominate your goals in life?
When making decisions, what priority do your physical desires
and the desires to satisfy your ego have?
What priority do the commands of God have?

1Jn 2:3,4 We know that we have come to know him if we obey his commands. The man who says, "I know him," but does not do what he commands is a liar, and the truth is not in him.

This is one of the external evidences that one has come to know Christ. Jesus said, "Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven." Mt 7:21 In fact referring to Jesus as Lord is not supposed to be simply an acknowledgement of his Diety, but rather it's supposed to  imply one's intention of doing what he says. Thus Jesus asks the rhetorical question, "Why do you call me, ‘Lord, Lord,’ and do not do what I say?" Luke 6:46

A person has not come to know Christ, let alone believe in him, if they have simply prayed a prayer to be saved without any actual intention of obeying him. That is not the kind of faith the qualifies one for salvation, as James writes in Jas 2:14, "What good is it, my brothers, if a man claims to have faith but has no deeds? Can such faith save him?" Apparently not!

1Jn 2:5,6  But if anyone obeys his word, God’s love is truly made complete in him. This is how we know we are in him:  Whoever claims to live in him must walk as Jesus did.

It is not that obedience indicates that our love is made complete, but rather that God's love for us drives us to obedience, so that the person who realizes God's love for the world will naturally obey God's commands. What kind of a lifestyle would be characteristic of such a person? A lifestyle which emulates Christ as he lived in the gospels. Not that we are called to wear robes and sandals, but rather there are allegorical links between Jesus's ministry and that which is the appropriate lifestyle for believers today.

For example in John 17 Jesus said of his followers, "they are not of the world any more than I am of the world." John 17:14 As Jesus was set apart for a mission in this world, we are also called to be on a mission, "to go and make disciples of all nations." Mt 28:19 We are called to serve. We are called to be holy. We can expect to be hated by the world as Jesus was.  For he said, "If you belonged to the world, it would love you as its own. As it is, you do not belong to the world, but I have chosen you out of the world. That is why the world hates you." There are many aspects of Jesus' attitude, mission, the events in his life, etc., find application in the Christian life.

1Jn 2:7-8  Dear friends, I am not writing you a new command but an old one, which you have had since the beginning. This old command is the message you have heard. Yet I am writing you a new command; its truth is seen in him and you, because the darkness is passing and the true light is already shining.

The commandment he is alluding to is of course John 13:34 where Jesus said, "A new command I give you: Love one another. As I have loved you, so you must love one another." Those who love fellow believers show themselves to be enlightened.

As for the "true light", John speaks of that elsewhere in John 1:9 saying, "The true light that gives light to every man was coming into the world." He's referring to Jesus. And Paul writes, "God, who said, Let light shine out of darkness, made his light shine in our hearts to give us the light of the knowledge of the glory of God in the face of Christ." 2Cor 4:6

1Jn 2:9-11  Anyone who claims to be in the light but hates his brother is still in the darkness. Whoever loves his brother lives in the light, and there is nothing in him to make him stumble. But whoever hates his brother is in the darkness and walks around in the darkness; he does not know where he is going, because the darkness has blinded him.

One of the most fundamental evidences distinguishing children of God from children of the devil which John speaks of throughout his letter is the evidence as to how one treats fellow Christians. There are false brethren in the Christian community who distinguish themselves by their unjustified hatred for legitimate Christians. Though they claim to be enlightened, and though they even claim to love fellow Christians, such people are lying if their lifestyle is inconsistent with their claims.

The reasons why they hate legitimate Christians is much the same as to why the world hated Christ. For example Jesus said to his unblieving brothers, "The world cannot hate you, but it hates me because I testify that what it does is evil." John 7:7 The most fundamental reason why false brethren hate legitimate Christians is because of sin. "This is the verdict: Light has come into the world, but men loved darkness instead of light because their deeds were evil. Everyone who does evil hates the light, and will not come into the light for fear that his deeds will be exposed." John 3:19,20

Therefore, "Have nothing to do with the fruitless deeds of darkness, but rather expose them. ...  But everything exposed by the light becomes visible" Eph 5:11,13 And by doing so you might just be able to distinguish genuine brethren from false brethen in your midst.

1Jn 2:12-14 I write to you, dear children, because your sins have been forgiven on account of his name. I write to you, fathers, because you have known him who is from the beginning. I write to you, young men, because you have overcome the evil one. I write to you, dear children, because you have known the Father. I write to you, fathers, because you have known him who is from the beginning. I write to you, young men, because you are strong, and the word of God lives in you, and you have overcome the evil one.

John is about to speak to them about their relationship with the world. But before doing so he clarifies here to whom he will be addressing his instructions. These are those of whom God is their Father, and who have known the true God, and whose sins have been forgiven and thus are in a right relationship with God, and who live by the Word of God and overcome the influences of the devil. May such be characteristic of all of us!

1John 2:15,16  Do not love the world or anything in the world. If anyone loves the world, the love of the Father is not in him.  For everything in the world— the cravings of sinful man, the lust of his eyes and the boasting of what he has and does— comes not from the Father but from the world.

We are not to love worldiness. We are not to conform our desires and goals to the sinful society in which we live, though the society rewards those who do so. The passions of the flesh and the praise of men, these are not what are to control the children of God. Rather we are to find our greatest satisfaction in doing the will of God. We are to seek the things which are eternal, as Moses did:

"By faith Moses, when he was come to years, refused to be called the son of Pharaoh's daughter; Choosing rather to suffer affliction with the people of God, than to enjoy the pleasures of sin for a season; Esteeming the reproach of Christ greater riches than the treasures in Egypt: for he had respect unto the recompence of the reward."Heb 11:24-26

1John 2:17 The world and its desires pass away, but the man who does the will of God lives forever.

"The day of the Lord will come like a thief. The heavens will disappear with a roar; the elements will be destroyed by fire, and the earth and everything in it will be laid bare. Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives as you look forward to the day of God and speed its coming. That day will bring about the destruction of the heavens by fire, and the elements will melt in the heat. But in keeping with his promise we are looking forward to a new heaven and a new earth, the home of righteousness." 2Pet 3:10-13

In view of these facts, how then shall we live? What should be our priorities in life?

"The time is short. From now on those who have wives should live as if they had none; those who mourn, as if they did not; those who are happy, as if they were not; those who buy something, as if it were not theirs to keep; those who use the things of the world, as if not engrossed in them. For this world in its present form is passing away." 1Cor 7:29-31

Everything else is secondary to seeking His Kingdom and His righteousness.