We know that the standard of God is high. Where man looks at outward acts / appearances and judges according to selfish measures, God looks at the heart and judges according to LOVE.
14 If we love our brothers and sisters who are believers, it proves that we have passed from death to life. But a person who has no love is still dead. 15 Anyone who hates another brother or sister is really a murderer at heart. And you know that murderers don’t have eternal life within them. (1 John 3:14-15, NLT)
It is impossible for us to truly love (according to God's measure of love) outside of Christ. That's how we can know for sure whether we really do have eternal life. When we are able to love the brethren, surely Christ lives in us and we are no longer dead in our sin. However, if we all too easily hate our Christian brothers and sisters, we are no better than a murderer! Christ cannot hate His own body; therefore, if Jesus lives in us, then it only follows that we too will love His Body.