Determination of Christianity for the Purpose of Deciding Biblical Behaviors Toward Individuals
We know we are Christians if we confess Master Jesus and believe the tenets of the gospel (Romans 10:9, 10, 13 and 1 Corinthians 15:1-8).
We know others are Christians if we hear them confess Master Jesus and if they say they believe the tenets of the gospel.
If we know someone is a Christian, then we behave toward them with all love and kindness and exhortation and admonishment and teaching, singing with them Psalms and hymns and spiritual songs, and confessing our faults to them, and shunning the ones living in some unrepentant sin after the first admonishment, then in front of two or three witnesses, and then in front of the congregation, so Satan can destroy their bodies and their spirits can be saved in the day of the Lord Jesus (2 Timothy 4:2; Titus 2:15; Colossians 3:16; Ephesians 5:19; James 5:16; Matthew 18:15-17; 1 Corinthians 5:5, 11, 13b).
If we know someone is not a Christian, then we behave toward them with wisdom and harmlessness (Matthew 10:16).