“In essentials, unity; in doubtful matters, liberty; in all things, charity.”
This statement is usually attributed to St. Augustine, yet no one has been able to point out just where in any of his works it is located. A general search on the quote reveals it being attributed to all sorts of figures in history.
The earliest Church document in which I am aware of it appearing is Pope St. John XXIII’s Ad Petri cathedram:
But the common saying, expressed in various ways and attributed to various authors, must be recalled with approval: in essentials, unity; in doubtful matters, liberty; in all things, charity (72).
Pope St. John XXIII merely referred to it as a common saying rather than attributing it to any particular saint or historical figure. And since no one else seems to have a definite source citation to point to, it would appear that its origins are unknown.