Summary: The danger of mislabeling a person as a Christian - when they are not
Mislabeling Leads to Negative Consequences
The English Standard Version of the Bible has this translation of Romans 9: 6b-8, "For not all who are descended from Israel belong to Israel, and not all are children of Abraham because they are his offspring... This means, it is not the children of the flesh who are the children of God..."
Modern day application for this passage of Scripture is God does not have grandchildren, nieces or nephews. You’re not a Christian because your parents or anyone in your family line is a Christian. You are not a Christian because you associate with church attendees. (Wuest's Word Studies in the Greek New Testament; Romans 4:1-8) Fair pretense and plausible profession may deceive men, but God cannot be so deceived; He sees through to what is really in the heart of mankind. True Christianity is born only when the Holy Spirit of Christ is allowed to do His work within the spiritual heart. (Matthew Henry Commentary; Romans 2: 28-29)
Jesus talked about blind guides and teachers who think they are instructing correctly, but are not living what they teach. He warns them, "...what the Father has not planted will be up rotted up..." (Matthew 15:11-14) The Apostle Paul in Romans exhorts Christians, "if you are convinced that your are a... light for those in the dark, an instructor... you who teach, do you teach yourself? ...As it is written; 'God's name is blasphemed among the Gentiles because of you.'" (Romans 2:19-24)
Nothing does the Gospel more harm than when people who are proclaiming to be Christians live and act like everyone else in this world does. Nothing is more disheartening than to find out that those who are claiming to be teachers of God's Word are living an extravagant lifestyle, are greedy for power and are self-focused. Tragically, false Christians and teachers continue to reappear through the ages and always find victims who are unable to discern false-hood from Biblical truth. The Apostle Paul explains it, "such people are false... deceitful workers, masquerading as apostles of Christ. And no wonder, Satan himself masquerades as an angle of light. It is not surprising, then, if his [Satan] servants also masquerade as servants of righteousness. False leaders seek to propagate their own version of the truth, having a form of godliness. Mislabeling leads the Biblical ignorant to living a life that displeases the Lord; they are unaware of it until the consequences are experienced.
In the farrier trade there are different sizes of horseshoe nails. For example, there are #5 Mustad and Capewell nails. Mustad's nails have little thicker shank while Capewell's shank is thinner. In the event that a #5 Mustad horseshoe nail is mislabeled as Capewell #5 and is driven into a thin hoof-wall, it is possible the horse's hoof-wall would crack or the horse would become nail-bound; this can result in causing a horse to become lame. An inexperienced farrier would drive the mislabeled nail into the horse's foot, not knowing his error. Although an innocent, misguided mistake, the horse still would suffer negative consequences of such a mislabeling of the horseshoe nails.