In a letter dated 24 September 1554, Knox received an invitation from a congregation of English exiles in Frankfurt to become one of their ministers. He accepted the call with Calvin's blessing. But no sooner had he arrived than he found himself in a conflict. The first set of refugees to arrive in Frankfurt had subscribed to a reformed liturgy and used a modified version of the "Book of Common Prayer". More recently arrived refugees, however, including Edmund Grindal, the future Archbishop of Canterbury, favoured a stricter application of the book. When Knox and a supporting colleague, William Whittingham, wrote to Calvin for advice, they were told to avoid contention. Knox therefore agreed on a temporary order of service based on a compromise between the two sides. This delicate balance was disturbed when a new batch of refugees arrived that included Richard Cox, one of the principal authors of the "Book of Common Prayer". Cox brought Knox's pamphlet attacking the emperor to the attention of the Frankfurt authorities, who advised that Knox leave. His departure from Frankfurt on 26 March 1555 marked his final breach with the Church of England.