How to tell if someone is a Christian, a Reflection on John 13:31-35

They had gathered for a meal, one that we know was their last meal together. Jesus insisted on washing their feet. At the meal, Jesus told them that one of them would betray him. When he identified Judas as the betrayer, Judas immediately went out into the dark (13:1-30).

Jesus speaks to the ones who remain.
"Now the Son of Man has been glorified and God has been glorified in him." Glorified, that is, made visible. It is now apparent that in Jesus, God is made visible to the world. Note the now.

"I am going to with you only a little longer. Where I am going you cannot come."
"I am giving you a new commandment: Love one another as I have loved you." Of course, this commandment was not new in the sense that they had never heard it before. See Leviticus 19:18 and Deuteronomy 6:4. What is new is that they will understand this commandment in terms of what Jesus does and what they will be willing to do.

He then told them the test by which Christians would be known--that they loved each other. Go back and read again: Now the Son of Man has been glorified and God has been glorified in him. By the love that Christians show each other, we can not only tell that they are Christians, we can see the glory of God.

Note: I am quoting generously from the commentary by Gail O'Day and Susan Hylen,