Martin Luther said, ‘There is not a word in the Bible which is extra crucem, which can be understood without reference to the cross.’
And J.I. Packer has written, ‘It cannot be over-emphasised that we have not seen the full meaning of the cross till we have seen it…as the centre of the gospel, flanked on the one hand by total inability and unconditional election and on the other by irresistible grace and final preservation.’
In the New Testament, the cross is:-
the subject of the eternal counsel of God, Acts 2:23.
the key to the Old Testament, Lk 24:26f; Acts 8:32-35.
the measure of human guilt, Acts 3:13-15.
the ultimate expression of divine love, Jn 3:16; Rom 8:32.
the supreme motive for the Christian life, Rom 12:1; 2 Cor 5:14f.
the central theme of Christian preaching, 1 Cor 1:18; Gal 6:14.
the highest note of praise and thanksgiving, Rev 1:5f.
Here’s what J.C. Ryle has to say about the centrality of the cross of Christ:-
The cross of Christ, – the death of Christ on the cross to make atonement for sinners, – is the centre truth in the whole Bible. This is the truth we begin with when we open Genesis. The seed of the woman brusing the serpent’s head is nothing else but a prophecy of Christ crucified. -This is the truth that shines out, though veiled, all through the law of Moses, and the history of the Jews. The daily sacrifice, the passover lamb, the continual shedding of blood in the tabernacle and temple, -all these were emblems of Christ crucified. -This is the truth that we see honoured in the vision of heaven before we close the book of Revelation. “In the midst of the throne and of the four beasts,” we are told, “and in the midst of the elders, stood a Lamb as it had been slain.” (Rev 5:6) Even in the midst of heavenly glory we get a view of Christ crucified. Take away the cross of Christ, and the Bible is a dark book. It is like the Egyptian hieroglyphics without the key that interprets their meaning, -curious and wonderful, but of no real use.
Let every reader mark what I say. You may know a good deal about the Bible. You may know the outlines of the histories it contains, and the dates of the events described, just as a man knows the history of England. You may know the names of the men and women mentioned in it, just as a man knows Caesar, Alexander the Great, or Napoleon. You may know the several precepts of the Bible, and admire them, just as a man admires Plato, Aristotle, or Seneca. But if you have not found out that Christ crucified is the foundation of the whole volume, you have read your Bible hitherto to very little profit. Your religion is a heaven without a sun, an arch without a key-stone, a compass without a needle, a clock without spring or weights, a lamp without oil. It will not comfort you. It will not deliver your soul from hell.
Mark what I say again. You may know a good deal about Christ, by a kind of head knowledge. You may know who he was, and where he was born, and what he did. You may know his miracles, his sayings, his prophecies, and his ordinances. You may know how he lived, and how he suffered, and how he died. But unless you know the power of Christ’s cross by experience, -unless you know and feel within that the blood shed on that cross has washed away your own particular sins, -unless you are willing to confess that your salvation depends entirely on the work that Christ did upon the cross, -unless this be the case, Christ will profit you nothing. The mere knowing Christ’s name will never save you. You must know his cross, and his blood, or else you will die in your sins.
J.C. Ryle, Old Paths, 248f.