Can you tell a true Christian by his actions? Jesus said that you can judge a tree by its fruit, so you should be able to. However, there must be some who profess Christ, but are not really saved, because Jesus mentioned that not everyone who calls him “Lord, Lord” will be with him in heaven. Only those who do the father’s will can be saved.
Some use the “no true Scotsman” fallacy to say that anyone who claims to be a Christian must be exhibiting the behavior of born-again Christians all the time. It goes like this. Suppose I say no Christian would do a certain thing. You might point out that you know a Christian who does exactly that. I say, “but no true Christian would do it.” Have I just made an error? Did I shift the meaning of the word “Christian?”
The “no true Scotsman” fallacy can’t be applied to Christianity. Being Scotch is a characteristic of birthplace or genetics. Being a “true” Scotsman is subjective. However, being a Christian is a choice of beliefs. It is something you can change. Also, because no one but God can see within the heart, no human can say whether anyone is a true Christian! Even if a Christian fails miserably in his faith and commits all kinds of sins, he may be dying inside, still holding to some shred of faith. Such people may even wish that other Christians would condemn the bad behavior and offer help. Conversely, there may be some who are regular churchgoers that hold inside deep feelings of bitterness, hatred and unforgiveness. They might not even believe in Jesus, though they would never admit it. They enjoy the church environment, because they consider it safe, and they wouldn’t want to lose all their nice church friends.
As another example, there are many people in jails now for violent crimes that they have never confessed to. Though we aren’t completely sure what they did, we are convinced they have some level of guilt sufficient to put them in prison. They will never confess or tell us all the details of what happened. How are we to differentiate these from prisoners who are innocent because they were framed?
Therefore, although Christians generally assume that all who claim to be Christians are true Christians, only God can be the judge. Saying the words “I believe Jesus is Lord” has no effect on your life unless you believe it in your heart.