An Anglican priest has unlocked the 270-year-old secrets of Charles Wesley's coded diary, throwing light on the turbulent relationship that he had with his brother John in the early years of the Methodist movement they founded.

Wesley, a central figure in the 18th century evangelical revival, has been portrayed as a spiritual giant of his times, and was the author of hymns including Hark the Herald Angels Sing. But his journals, partly written in a personal shorthand, have revealed him to be a depressive character who was prepared to go to extreme lengths to wreck his brother's proposed marriage and who was ready to blame him for his own wife's miscarriage.

The Rev Professor Kenneth Newport, Pro-Vice-Chancellor at Liverpool Hope University, was the first to crack the cipher and spent nine years transcribing the 1,000-page hand-written manuscript held at John Rylands Library in Manchester.

The “hidden” material offers an insight into Wesley's fierce determination to prevent the Methodist societies from breaking away from the Church of England, and disagreements with his more influential older brother.

Wesley's shorthand, which omits vowels and abbreviates consonants, is a highly personalised adaptation of that invented by John Byrom, the 18th century poet, diarist and stenographer. Byrom, whose method was taught at Oxford University, published his New Universal Shorthand in 1740. Wesley's is severely abbreviated, sometimes using a string of consonants without breaks. Whole sentences are elided and the spellings are often phonetic. The language generally is that of an 18th century gentleman and preacher.

“The problem is not so much transcribing the letters, although that can be tricky, but more often than not you end up with a skeletal construction because he does not put in vowels,” Mr Newport said. “He abbreviates very severely, much more so than one might expect.”

He was confused, for example, by the repetition of “hr” until he concluded that the letters stood for holy writ. The breakthrough came when he discovered that Wesley had rendered part of the scriptures in shorthand and was able to compare the abbreviations against the King James Bible. “I was determined to unlock it. Charles was a great man, with insights that remain important for us today,” he said.

Wesley's journals open with a description of his arrival in America in 1736 and end in 1756. No journal survives from his later life, although it is likely that he continued to write down his thoughts until his death in 1788. He lapsed into shorthand when he wrote about sensitive subjects, such as his disputes with John Wesley over the future direction of Methodism, and his vehement disapproval of his brother's proposed marriage to Grace Murray.

The journals have uncovered that the two brothers, by then middle-aged and attracting unwanted attentions from the movement's “sisters”, agreed that neither would marry without the other's approval. Wesley wrote in extreme terms about discovering that his brother had made overtures to Miss Murray: “He is insensible of both his own folly and danger, and of the divine goodness in so miraculously saving him.” Wesley also suggested that the dispute with his brother was behind his wife Sally's miscarriage.

Historians have debated why Charles was so set against the match but Mr Newport suspects that it was simply because he regarded Grace Murray as beneath their social station.

Slander and ingratitude - a translation of a page of Wesley's shorthand journal

Monday, March 22 [1736] While I was persuading Mr Welch not to concern himself in this disturbance, I heard Mrs Hawkins cry out: “Murder!” and walked away. Returning out of the woods, I was informed by Mr Welch that poor blockhead Mrs Welch had joined with Mrs Hawkins and the Devil in their slanders of me. I would not believe it till half the town told me the same, and exclaimed against her ingratitude.