The Didache is, in all probability, the oldest surviving piece of non-canonical literature that we have access to today. It is not so much a letter as a handbook for new Christian converts, consisting of instructions derived directly from the teachings of Jesus.
As he began the last year of his life, he wrote, ‘To all appearance the present year will be more perilous than any I have seen, but if I live to complete the Persian New Testament, my life after that will be of less importance. But whether life or death be mine, may Christ be magnified in me. If he has work for me to do, I cannot die’.