Biblically, a Christian is someone who has received the Lord Jesus as Savior (John 1:12), trusts Him alone for the forgiveness of sins (Acts 4:12), has put no trust in his/her own efforts (Isaiah 64:6) to please God, and repented from his/her sins (Mark 1:15).
Experimentally speaking, the life of a Christian does not consist only of Biblical head- knowledge. It is doctrine that defines who Jesus is and what He has done, but it is not the end of all things. We are Christians who believe the above points, yes, but we have a living and open relationship with the Lord Jesus. We have by faith reached out and trusted in Him, and have experience Him through His indwelling Spirit.
Coming to Christ is where it begins, but becoming a Christian is not where it stops. As Christians, we seek to do the will of the Lord, to follow in His footsteps, and to honor and glorify God in all we do.