A christian is someone who follows Christ, by definition of terms. But a practical meaning to me is someone who trusts in Jesus Christ as their Lord and Savior, but the only way for us to know is to see evidence through actions and spiritual fruit. So you and I do not know if someone else is a christian, we can presume given the evidence, but we do not know. Only GOD knows the motivation of the heart.
Also here is a good start,
Romans 3:23 "For all have sinned and fall short of the glory of God."
Romans 6:23 "The wages of sin is death, But the gift of God is eternal
life through Jesus Christ our Lord."
Romans 5:8, "God demonstrates His own love for us, in
that while we were yet sinners Christ died for us!"
Romans 10:13 "Whoever will call on the name of the Lord will be saved!"
Romans 10:9,10 "...If you confess with your mouth Jesus as Lord, and believe in your heart that God raised Jesus from the dead, you shall be saved; for with the heart man believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation."