1 John 3:7-10
Kids, don’t let anyone fool you. People who practice doing right the way Jesus did right are righteous. People who do wrong are the devil’s children. The devil has been doing wrong from the beginning, but the child of God appeared to put the devil out of business. People who are God’s children don’t do wrong, because God’s nature is their nature. They can’t do wrong because they’re Gods children. So you can tell the children of God from the children of the devil, because if they’re not doing right, and if they don’t love their siblings, they’re not God’s.
This is the theological duck test.
If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.
For all the theological wrangling over propositions and faith statements and right belief, the crux of the matter comes down to whether someone is in the habit of doing right or not. If they’re righteous, then there is something of God in them. If they’re bent toward wrong, then it makes no difference what they say they believe in.
“But how does one know what is right and wrong?” you ask. John’s letter doesn’t pretend to lay out a system of ethics. John doesn’t go into much detail or lay out lots of rules. There’s just one clue. You can tell someone’s doing right by how they treat their siblings. Siblings broadly understood. Brothers and sisters of the human race. Is it loving? If so, it’s right.
Beyond that, the terms to describe love are left for you to work out. But there’s another test for that: the elephant test.
It is difficult to describe, but you know it when you see it.