How does the New Testament define a brother in Christ? According to the New Testament, what makes a human a brother in Christ? And also, according to the New Testament, which people are definitely not Christians, definitely not brothers in Christ?
I would want to have a set of key critical conditions-prerequisites (elicited from the New Testament) that would be necessary conditions for a person to be considered a Christian or a brother in Christ (I mean, if a person lacks at least one of those conditions, he is definitely not a Christian and definitely not a brother in Christ)
(I am not requesting here others' personal views. I really want to know how this very important matter is presented in the New Testament.
This matter is in fact a fundamental matter pertaining to Christian faith. It's not like "Is smoking cigarettes good or bad according to the New Testament?" While smoking may not be an important issue in Christianity, the matter that I am concerned about here definitely is)