What is the measure of faith in Christ? 1 John 1-3

1 John 3:10
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.

John is sometimes a confusing writer to read.   His truths are so plain, once we grasp them, but he writes like your ascending to the truth on a spiral staircase, continually twisting and turning, while seeing the same scene above and below at different angels and different views.   Yet, John gives great, big, bold statements and when we land on them they are easy to grasp and safe to hold on to and wonderful for meditation.   The above passage is an example of such a "landing."   In the third chapter of John he is addressing the temptation and false teaching in the church that you can both believe in Jesus Christ AND continue to sin.   The argument of false teachers is that Christ saves the soul but the body is not saved and continues in sin, and that is okay since it is not part of the redemptional plan.   So, to the false teachers, sinning is something we live with and really doesn't equate in the redemption process.   John is debunking that argument in the third chapter and he has already told them that those who "practice sin" (meaning continued and repetitive and un-repentive) are not really believers in Christ.   He is not saying believers don't sin, but he is telling them (and us) that those who sin and make a practice of it, do not have a relationship with Christ.   In the middle of that argument, he lands at the above verse.    Like a summary statement to his apology, John tells them (and us) that the easiest way to know your relationship with Christ, whether it is genuine or fake, is your repetitive walk in God's righteousness, demonstrated by whether he or she loves his brother.   The litmus test for believers is their walk in God's way and their love of other brothers and sisters in Christ.   We can't hate or mistreat other brothers in Christ and say we are believers.   That just doesn't work.   It is, for sure, the hardest aspect of the Christian life that we have failed relationships with our brothers and sisters in Christ.   To love them, in Christ, is often a struggle and beyond our normal fleshly tendencies.   God has instructed us to love our brothers and sisters in Christ through Jesus and in His power.   That is part of the exchanged life.   We give Him our sin to pay for and satisfy the penalty and He gives us His love to shed abroad in our hearts to others.   God's righteousness is seen in how we treat other children of God.    God's righteousness is demonstrated when we love those who aren't always lovable.  God loves us when we are not always lovable (we are never lovable!).    We ought not measure our Christianity by our church attendance, level of service, brand of worship or even depth of knowing God's Word.   If all of that is not worked out in our love for others and demonstration of that love through practical manners, we have lost sight of true faith and true belief.