Answer: Since God loved us, we ought also to love one another (verse 11). God has give us this command: Whoever loves God must also love his brother, (verse 21).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
A very applicable commentary: "If anyone says, ‘I love God,' imagining that his religious feelings of adoration and gratitude are of a higher order than ‘love with actions and in Truth,' while at the same time he ‘hates his brother,' consciously wishing him evil, ‘he is a liar'--he is telling and living a lie, and would realize that it is so if only he would stop to consider. For ‘the brother whom he has seen,' in all his sinful weakness, is the appointed receptacle for the divine love which he professes to be receiving."
Now, take the above commentary in the reverse. YOU are the brother/sister whom YOUR brethren have seen, and in all of YOUR sinful weakness, YOU are the appointed receptacle for the divine love, which YOUR brethren profess to be receiving and living in from God.
Matthew 7:12: [Jesus said,] "So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets."
Now consider the following:
Matthew 25:40, 45: "The King will reply, ‘I tell you the Truth, whatever you did for one of the least of these brothers of Mine, you did for Me....' He [the King] will reply, ‘I tell you the Truth, whatever you did not do for one of the least of these, you did not do for Me.'"
1 John 3:16-20: "This is how we know what love is: Jesus Christ laid down His life for us. And we ought to lay down our lives for our brothers. If anyone has material possessions and sees his brother in need but has no pity on him, how can the love of God be in him? Dear children, let us not love with words or tongue but with actions and in Truth. This then is how we know that we belong to the Truth, and how we set our hearts at rest in His presence whenever our hearts condemn us. For God is greater than our hearts, and He knows everything."
In the same way that we are not to "love with words or tongue but with actions and in Truth" by demonstrating (manifesting) our love for one another, we are to demonstrate our faith by our deeds (actions) and we are to demonstrate our obedience by our actions:
James 2:14-24: "What good is it, my brothers, if a man claims to have faith but has no deeds? Can such faith save him? Suppose a brother or sister is without clothes and daily food. If one of you says to him, ‘Go, I wish you well; keep warm and well fed,' but does nothing about his physical needs, what good is it? In the same way, faith by itself, if it is not accompanied by action, is dead. But someone will say, ‘You have faith; I have deeds.' Show me your faith without deeds, and I will show you my faith by what I do. You believe that there is One God. Good! Even the demons believe that--and shudder. You foolish man, do you want evidence that faith without deeds is useless? Was not our ancestor Abraham considered righteous for what he did when he offered his son Isaac on the altar? You see that his faith and his actions were working together, and his faith was made complete by what he did. And the Scripture was fulfilled that says, ‘Abraham believed God, and it was credited to him as righteousness,' and he was called God's friend. You see that a person is justified by what he does and not by faith alone."
James 1:22-25: "Do not merely listen to the Word, and so deceive yourselves. Do what it says. Anyone who listens to the Word but does not do what it says is like a man who looks at his face in a mirror and, after looking at himself, goes away and immediately forgets what he looks like. But the man who looks intently into the perfect law that gives freedom, and continues to do this, not forgetting what he has heard, but doing it--he will be blessed in what he does."
So you see that faith (belief), obedience and love without a demonstration (manifestation) of such are DEAD. Because we are children of the living God, we, too, live--we are not dead children! We have no life of ourselves:
Colossians 3:2-4: "Set your minds on things above, not on earthly things. For you died, and your life is now hidden with Christ in God. When Christ, WHO IS YOUR LIFE, appears, then you also will appear with Him in glory."
Having been brought from death to life by our rebirth through the blood of His Son and because we live in the Source of all life and love, it follows that our faith, obedience and love are to be actively manifested in everything we do and in how we behave towards our brethren, because we are living children of God! If this is not accomplished in us, then the same thing can be said of us that Christ said to the Christians in the church at Sardis in Revelation 3:1-3:
"These are the Words of Him Who holds the seven spirits of God and the seven stars. I know your deeds; you have a reputation of being alive, but you are dead. Wake up! Strengthen what remains and is about to die, for I have not found your deeds complete in the sight of My God. Remember, therefore, what you have received and heard; obey it, and repent. But if you do not wake up, I will come like a thief, and you will not know at what time I will come to you."
