On my occasional visits to Westminster Cathedral, I sometimes pause to watch other visitors as they arrive. Regulars stride, scurry or amble in, according, I suppose, to temperament or purpose. Newcomers, on the other hand, advance more cautiously.

Understandably so. I am not sure there is anything quite like this experience in London, at least on this scale. In the matter of a few, short, unimpeded steps – no queue, no ticket checking, no bag search – you can leave the Earthly City and enter an approximation, at least, of the City of God. Sometimes it looks as if new visitors edge backwards ever-so-slightly. People know when they’re not in Kansas any more.

However, once the sight has acclimatised to the overall effect of the gloomy domes above, the variously coloured marble pillars all around and the statues and mosaics sprouting left and right, it rises inexorably upwards to the dominant feature of the cathedral’s interior. As Pope Benedict observed during his homily here in 2010, the visitor “cannot fail to be struck by the great crucifix dominating the nave, which portrays Christ’s body, crushed by suffering, overwhelmed by sorrow, the innocent victim whose death has reconciled us with the Father and given us a share in the very life of God. The Lord’s outstretched arms seem to embrace this entire church …”

The crucifix, or Great Rood, has been in place since 1903. It is 30ft high and 23ft wide and hangs above the junction between the nave, sanctuary and transepts. It was designed by the cathedral architect JF Bentley, and carved by Charles Beyaert in Bruges. William Christian Symons did the painting in which the figure of the crucified Christ is entirely surrounded by red. There are end panels on each limb of the cross carrying the symbols of the four Evangelists who described the crucifixion. On the reverse, visible to the priest saying Mass, is a depiction of our Lady of Sorrows.

The impact is redoubled, in my eyes, by the mosaic in the arch between the sanctuary and the apse, which, when one looks down the nave from the great door, acts as the backdrop to the cross. In Building of Faith, John Browne and Timothy Dean describe Christ “now in the heavenly sphere surrounded by the evangelists in symbolic form and contemplated by the twelve apostles in glory. Christ holds a chalice of the blood of redemption against a background of myriad faces of the just in graduated circles of blue.”

One is thus presented with a powerful sequence: from the Christ of blood and suffering to Christ in majesty, a sequence signalled in colour terms with the movement from vermillion to azure. In the archway, one sees Christ, who had been given vinegar for his thirst on the cross, now serene and triumphant, and surrounded by what looks like a calm, clear, cool sea.

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