If we walk in light
"This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all.
If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.
But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin. 1John 1:5-7.
Here is a sign of a true Christian. They walk in the light of God. As the apostle Paul states they show the fruit of the spirit. Paul writing,
"But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law." Galatians 5:22,23.
As apposed to the acts of the sinful nature that Paul says is,
“The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like.”
By watching what a person does you can clearly distinguish who is truly a Christian, a follower of Christ Jesus and who is not.
True Christians always do their best to their best to walk in the light of God.
Please think about it.