The actual quote; "In necessariis unitas, in dubiis libertas, in omnibus caritas", is literally translated as "in necessary things unity; in uncertain things freedom; in everything compassion". It was never written by St. Augustine. It was in fact spoken by Marco Antonio Dominis, a 16th century Croatian apostate in support of the Reformation. The attribution to St. Augustine was yet another fabrication by John Wesley.
It does beg the question as to what is necessary and what is uncertain, though. The notion that unity is more important than communion and doctrine is only important to the many disparate factions who were more concerned with what it takes to BE Protestant and united in opposition to the Church than to BE Christian.
It does not matter the degree to which "faithful" Catholics believe or follow it. For Catholics liberty means the freedom to do what we ought, not what we want.
Peace be with you.
But used by Pope John XXII in LITTERAE ENCYCLICAE AD PETRI CATHEDRAM 1959
[official translation]
Religious Controversy 71. The Catholic Church, of course, leaves many questions open to the discussion of theologians. She does this to the extent that matters are not absolutely certain. Far from jeopardizing the Church's unity, controversies, as a noted English author, John Henry Cardinal Newman, has remarked, can actually pave the way for its attainment. For discussion can lead to fuller and deeper understanding of religious truths; when one idea strikes against another, there may be a spark.(25)
72. 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.
we find the "must recalled with approval" usage of the "apostate's" invention!
John XXIII in the preceding paragraph citing Cf. J.H. Newman, Difficulties of Anglicans, vol. I, lect. X, p. 261s.