"By this all people will know that you are my disciples, if you have love for one another.” -John 13:35
How do you read this verse? Is a person revealed to be a Christian by his loving behavior? Compared to Jesus, I fail at this. How about you? In the Greek text, as in some English translations, the word love is a noun. Therefore every true Christian possesses a thing called love (agape). What is that thing? I would like to suggest that it is something like a new way of thinking or an imprint on the mind or heart. It is an attraction to righteousness as perfection. The scripture might call it the testimony of righteousness.
Every Christian has it and no non-Christian has it. The trick in identifying it is to get past Sunday School answers and sanctification jargon. While you are actively perceiving a person as righteous in Christ without personal works, does your heart sing or scowl? Non-Christians commonly tell me that they have never even had the thought that righteousness didn't require works. Since church people are commonly trained to appeal to sanctification for proof of justification, patience is needed to get below the surface to hear if a person is trusting in Christ alone for their justification with God.