A true Christian is a person who has put faith and trust in the person and work of Jesus Christ, including His death on the cross as payment for sins and His resurrection on the third day. John 1:12 tells us,

“Yet to all who received Him, to those who believed in His name, He gave the right to become children of God.”
 
The mark of a true Christian is love for others and obedience to God’s Word.  First John 2:10 tells us, 

"Whoever loves his brother abides in the light, and in him there is no cause for stumbling."

A true Christian is indeed a child of God, a part of God’s true family, and one who has been given new life in Jesus Christ.  First John 3:1-2 tells us, 

3 See what great love the Father has lavished on us, that we should be called children of God! And that is what we are! The reason the world does not know us is that it did not know him. 2 Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when Christ appears, we shall be like him, for we shall see him as he is.

As Christians we are a Fellowship of Believers in Jesus Christ.  Acts 2:42 tells us, 

"They devoted themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer. "