What Is Christ to the Christian?
Christ must be central to Christians, not morals, obedience, surrender, love, kindness, or any other kind of work. Without Christ, there is no Christian.
When Christ is central, it means He is our supreme desire. We want Him and Him alone. We trust Him and Him alone. Everything about our lives intersects at Christ. He is the apex of our lives.
Christians are Christ-centered because justification, sanctification, and glorification only happen in and through Him. He is, as the writer of Hebrews makes clear, our great high priest. He spans the gap between God and the sinner. For their salvation, He atones with His own blood. For their maturity, His Spirit works in them to make them holy. For their eternal glory, He presents them before the Father in His own robes of righteousness.
Without Christ as the main course of the Christian life, the Christian is prone to stray from the path into the ditch. In reality, Christ as anything but the main course is what you find in the ditch. Either side of the path bodes poorly for the traveler. To the left are moralism (hyper-law) and legalism. To the right are anti-law (antinomianism) and hyper-grace; both of these result in a Christ-less salvation which is no salvation at all.
Of course, the result is easy to understand for those who fall to the left. Legalism sneaks the law into grace, making the grace of little to no effect. Legalism is a 'grace plus' kind of Christianity. When any condition is added to the atonement of Christ, or when any work to the righteousness of Christ is added, Christ is not fully trusted and the gospel not fully embraced. If anything is added to our salvation, even the smallest detail, it becomes salvation based on our work, not Christ's.
For those who fall right, the destination of a Christ-less salvation is not as obvious – that is until they land. At the onset, those who promote grace and deny law seem to be on to something good and true. But as they progress, it becomes clear that they have simply fallen into the massive web of a 'grace plus' system. This time, the law is not added to grace. Instead, grace is expanded to the point that it becomes disfigured and unscriptural.
Free from the law, O blessed condition I can sin as I please and still have remission
The claim that grace does not change the individual is Christ-less thinking. To say so may not add anything to grace, but it surely takes much away. A common saying like, "God accepts me just as I am," is unhelpful at best. Christ-centered thinking would rather say, "God accepts me regardless of who I am because He accepts me on the basis of Christ's work. He accepts me but will not leave me in my sin. Instead, He will conform me into the image of Christ."
Christ commands our attention throughout our lives and to the end of eternity. He is the one with whom we must be in continual fellowship. Jesus' own words, "Abide in me," ought to propel us away from ourselves, our works, and all the world's attractions, and propel us into His arms. Charles Spurgeon brings a compelling case for the importance of Christ-centered Christianity:
Communion with Christ is a certain cure for every ill. Whether it be the wormwood of woe, or the cloying surfeit of earthly delight, close fellowship with the Lord Jesus will take bitterness from the one, and satiety from the other. Live near to Jesus, Christian, and it is a matter of secondary importance whether thou livest on the mountain of honour or in the valley of humiliation. Living near to Jesus, thou art covered with the wings of God, and underneath thee are the everlasting arms. Let nothing keep thee from that hallowed intercourse, which is the choice privilege of a soul wedded to the well-beloved. Be not content with an interview now and then, but seek always to retain his company, for only in his presence hast thou either comfort or safety. Jesus should not be unto us a friend who calls upon us now and then, but one with whom we walk evermore. Thou hast a difficult road before thee: see, O traveller to heaven, that thou go not without thy guide. Thou hast to pass through the fiery furnace; enter it not unless, like Shadrach, Meshach, and Abednego, thou hast the Son of God to be thy companion. Thou hast to storm the Jericho of thine own corruptions: attempt not the warfare until, like Joshua, thou hast seen the Captain of the Lord's host, with his sword drawn in his hand. Thou art to meet the Esau of thy many temptations: meet him not until at Jabbok's brook thou hast laid hold upon the angel, and prevailed. In every case, in every condition, thou wilt need Jesus; but most of all, when the iron gates of death shall open to thee. Keep thou close to thy soul's Husband, lean thy head upon his bosom, ask to be refreshed with the spiced wine of his pomegranate, and thou shalt be found of him at the last, without spot, or wrinkle, or any such thing. Seeing thou hast lived with him, and lived in him here, thou shalt abide with him forever.
What is of ultimate importance in this life? What will bring true comfort, joy, and peace? Who will clear the guilt from our conscience? What will last forever? It is Christ, and Christ alone.
 Spurgeon's Morning and Evening Devotional. March 9, Evening.