"By this the children of God and the children of the devil are obvious: anyone who does not practice righteousness is not of God, nor the one who does not love his brother." (1 John 3:10)We cannot know what is in another person's heart, but we can see the outward effluence of it that gives evidence to what is inside. Jesus, speaking of false prophets, said, "You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they? So every good tree bears good fruit, but the bad tree bears bad fruit. A good tree cannot produce bad fruit, nor can a bad tree produce good fruit." (Matthew 7:16-18) The proof of a person's life is demonstrated by the fruit they produce. A good life will produce good fruit and a bad life bad fruit. To be able to distinguish between the children of God and the children of the devil we need only to look at the fruit of their lives.
In particular, John draws our attention to two specific fruits that all believers should bear. First, while before we practiced sin, as a child of God we now practices righteousness. Those who have not yet come to Christ, while they may desire to practice righteousness, are incapable of it for they are slaves of sin. This does not mean that they will not occasionally stumble and fall into righteousness just as the children of God occasionally stumble and fall into sin, but it does mean that they will never sustain a life built upon the consistent and willful practice of what is right. Righteousness is born of God and is a result of the seed of God being birthed within is. Furthermore, righteousness is more than simple deeds done in conformance to the law. It is the good works that are born out of a purified heart, conceived in love, and performed in faith. Paul reminds us that, "In Christ Jesus neither circumcision nor uncircumcision means anything, but faith working through love." (Galatians 5:6) Righteousness is more than doing what's right, its doing what's right for the right reasons. Its not obedience to the law but actions of faith working through love.
Secondly, John reminds us that our love for God can never be separated from our love for one another. God created us all and we are all made in His image. Furthermore, God loves all He created and we are all individually loved by God. How them, can those who love God hate those whom He loves? When we hate those whom have been created in the image of God, we hate from something within us that is foreign to God. Our hate is birthed from the seed of Satan and not the seed that is from God. In our relationships with each other we may experience a wide range of emotions, from joy, pride, comfort, and even longing to anger, frustration, impatience, and even disappointment. However, our emotions should never lead us into hate. Hate blinds our eyes and darkens our lives to the point where we no longer see clearly to guide our own lives let alone someone else's. Our emotions do not define us but rather the determination of our heart and our actions towards one another define who we are. Those who are born of God have the very God of love living in their hearts. How can they but help to bring forth the fruits of love towards their brothers, their sisters, and all of God's creations?