Didymus was a leading opponent of Arianism, which held that Christ is not truly divine but a created being. He was, however, also a follower of Origen and took on some of his errors, so that, although he was not condemned by name at the time that Origenism as a whole was condemned, his works fell out of fashion and, as they were not copied, gradually disappeared over the centuries.

  Didymus’ Trinitarian and Christological doctrine is, however, perfectly orthodox. He was a pioneer in expressing the doctrine of the Trinity in a way that was clear and unambiguous and could not be misunderstood. This was a delicate business requiring a careful choice of terms, especially in Greek, which lacked a direct equivalent of straightforward Latin words such as persona. “On the Trinity”, which is used in the Office of Readings, is his most important work.

  In the earliest centuries all vestments were white – the white of baptismal purity and of the robes worn by the armies of the redeemed in the Apocalypse, washed white in the blood of the Lamb. As the Church grew secure enough to be able to plan her liturgy, she began to use colour so that our sense of sight could deepen our experience of the mysteries of salvation, just as incense recruits our sense of smell and music that of hearing. Over the centuries various schemes of colour for feasts and seasons were worked out, and it is only as late as the 19th century that they were harmonized into their present form.