Completed in 1916, the Tympanum is a key feature of the Cathedral facade. The detail of a photograph below from 1908 shows the entrance before the mosaic was added.
The completed mosaic is based on a pencil sketch by the Cathedral architect, John Francis Bentley (below) - a unique example of his designing mosaic decoration. In the orginal drawing, Christ is enthroned, displaying his wounds ( a reference to the dedication of the Cathedral to the Precious Blood), while around him the other saints to whom the Cathedral is dedicated: St Peter (patron of Westminster) , Our Lady, St Joseph and St Edward the Confessor.
Bentley's design was later worked up by his successor as Architect, William Marshall, and the image below appeared in the Westminster Cathedral Chronicle of 1907. This was more sumptuous than Bentley's original drawing. However, the mosaic was not executed until 1916.
The artist was Robert Anning Bell, and it is said that Cardinal Bourne was disappointed with the result; he beleived that Bell had paid little attention to his views and had departed from Bentley's design. In the work as executed, the composition is simplified and austere, and Christ's wounds are not visible. Instead he holds a book bearing the words 'I am the gate; if anyone enters by me he shall be saved.' (John 10:9) - a new theme unrelated to the dedicatory inscription in Latin above.
There was much comment at the time that the background should be gold (an opinion with which I agree); however, it was explained that a white background would better relate the image to the colouring of the facade, and that gold on a flat surface would not reflect light as it would on a concave recess (as at San Marco's in Venice). It was also believed that a starker design would stand more chance of surviving the London grime and smogs.
It is said that Cardinal Bourne remained unconvinced by the executed design, believing that Anning Bell (who was a non-Conformist) had been unable to comprehend properly Catholic iconography. Henceforth, the Cardinal resolved, only Catholic artists would be commissioned.