John writes in 1 John 2:3-6, 9-11
We know that we have come to know him if we keep his commands. 4 Whoever says, “I know him,” but does not do what he commands is a liar, and the truth is not in that person. 5 But if anyone obeys his word, love for God is truly made complete in them. This is how we know we are in him: 6 Whoever claims to live in him must live as Jesus did.
Anyone who claims to be in the light but hates a brother or sister is still in the darkness. 10 Anyone who loves their brother and sister lives in the light, and there is nothing in them to make them stumble. 11 But anyone who hates a brother or sister is in the darkness and walks around in the darkness. They do not know where they are going, because the darkness has blinded them.
The claim to be a Christian may be variously stated in terms of knowing God (vs. 4), or living in Christ (vs. 6), or being in the light (vs. 9), but invariably, if it’s an authentic claim, it will show itself in a new life of obeying God (vs. 4), imitating Christ (vs. 6), and loving our brothers and sisters (vs. 9-10). Without such a moral authentication, the claim is seen as bogus.
To which someone might reply, “Because no one is perfectly obedient then no one knows God!”
Wrong. As Calvin said,
He does not mean that those who wholly satisfy the law keep his commandments (and no such instance can be found in this world), but those who strive, according to the capacity of human infirmity, to form their life in obedience to God.