This question seems like a question that only an unbeliever would ask. Most believer would never ask this question because they fill like they already know the answer. In fact, we Christians feel like it is abnormal to have a mature Christian ask this question. But ever since his conversion, Paul spent the rest of his life asking this question. Much so that even at his old age his mind bungled with this question he cried out and pray, "that I may know him...". So who is this Christ? As professing Christians do we really think we've grasp the height, length, breath and depth of the answer of this question?
First of all, Christ is not just a man who live at a time in history. He is not just an historical personality who redeemed us by his blood and gave us salvation. Paul made a statement in which we need to take into deeper meditation. He said, "Therefore, from now on, we know no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer." (2Cor 5:16). Paul was saying we use to know Christ according to the flesh but now we know him according to the spirit. We know him as a man who did some really great things but now we are knowing him better, we are knowing him as spirit and life.
Look very closely at the early history of the Church and her understanding of Christ, she only knew him according to the flesh. Look at the first three books of the Gospel they talk about Christ according to the flesh. It was all about this man in history that did some amazing things. But when you come to the Gospel of John and the epistles of Paul especially Ephesians and Colossians we find a presentation of Christ according to the spirit. Christ wasn't just a man or a Son of God, he is the Life, the Way, the Word, the Truth, the Water, the Righteousness, the power and wisdom of God, redemption, salvation sanctification, atonement etc. Christ is our home "for in him we live, and move, and have our being;" (Acts 17:28). Christ is our Land for we are "rooted and built up in him," (Col 2:7). He is our all for our "life is hid with Christ in God" (Col 3:3). Christ is no longer just a man in history but he is the life in us and the atmosphere around us. He is in, through and around us.
Who then is this Christ according to the spirit? What is the height, length, breath and depth of this Christ? What is in this Christ, what constitute this Christ? In the New Testament phrases like "in Christ" appears 130 times. Paul was always talking about "in Christ", because he was constantly search for what is in Christ - what is the spiritual anatomy of Christ. But after the death of the apostles Christians became interested in many other things but Christ. They already know the historical Jesus and they are satisfied with that. They do not find the essence of knowing Christ according to the spirit neither are they concern with exploring that which is in Christ. But we are called to be a spiritual people and we ought to seek to know Christ after the spirit.
Who is Christ? Is this a normal question for a Christian to ask? Of course yes. It is not just normal but it is the only question that should be in our minds as Christians.