Almost everything associated with this literature reached the modern world in 1945. Pistis Sophia did not. The Askew Codex, a parchment book of some three hundred and fifty pages, was acquired by the British Museum in 1785 from the estate of Anthony Askew, a London physician and bibliophile; where he obtained it is not known. It has been in British public hands ever since, and is now in the British Library.
That accident of chronology matters for how the text has been used. A Latin translation appeared in the 1850s, German and French versions followed, and by the later nineteenth century Pistis Sophia was the single work by which educated readers outside the specialist literature knew anything at all about this kind of Christianity from the inside. Everything else came filtered through the church fathers who attacked it. For roughly a hundred and sixty years, this book carried that weight almost alone, and a great deal of what was written about the movement before the Second World War is in effect generalisation from one late and unrepresentative manuscript.
The codex itself is usually assigned to the fourth century. The Coptic is a translation from a Greek original, and the composition is normally placed in the third century, with the fourth book widely thought to be earlier than the first three and sometimes separated off as a distinct work. That makes the text late by comparison with most of the Nag Hammadi material, whose underlying Greek often reaches back into the second century. It is a product of a developed Egyptian tradition rather than of its formative period, and it should be read as such.