I happened to be reading LP Cruz's blog today, and noticed an article in which he ascribed to "a 17th century Lutheran pastor" the famous saying, "In the essentials, unity, in the non-essentials, liberty, and in all things, Charity," or something to that effect.
It seems like sound-thinking and appeals to our softer side; but who said it first? LP Cruz's post sparked my interest to dig deeper into the subject.
Ramage's "Beautiful thoughts from Latin Authors," ascribes the maxim to Melancthon, and notes that it has been carved in stone above his garden gate. (link) Cox too seems to maintain Melancthon as the originator, posting that quotation prominently on the title page of his biography of that reformer (link). Matthes likewise seems to be of the same view in his life and works of Melancthon (link), though my German and ability to read the older scripts is not good enough for me to be definitive. Hoefer also seems to be willing to attribute the saying to Melancthon (link).
Belton thinks that the origin of the phrase is "really unknown," though he seems to have found it in a few 17th century writers (link). This is perhaps the most honest and direct conclusion we could give, but it is our part as scholars to dig, guess, and delve. To that end, we cannot be satisfied with Belton's willing agnosticism on the matter.
Remarkably, the earliest I was able to find a reference (published 1719) to this famous maxim was in the works of the unworthy son of Francis Turretin, Jean Alphonse, who provides the saying with the addition of Prudence to the final line of the saying (link). J.A. Turretin appears to ascribe the phrase to Witsius. Cunningham (and others) agree that Witsius adopted this as his favorite motto (link).
Meldenius - not Augustine
Stanley ascribes the quotation to Rupertus Meldenius, and notes that it had for a time been falsely ascribed to Augustine (link). Jones notes the dubious ascription to Augustine here as well (link). Hamerton appears to have bought the Augustine line (link).
Narrowing it down Further
Hoyt lists both Meldenius (his preference) and Melancthon (link) (much the same thought here, as well).
Augustine was a rather obvious misattribution, his weighty name getting the credit for anything good in Latin among many Protestants. Ironically, the saying eventually came to be approved by a pope, as this thoughtful web page noted (link) and seems to have become taken essentially as dogma in other Catholic writings (e.g.). This may perhaps have been due to its misattribution to Augustine, though the pope seemed to have been aware of the dubious origin of the maxim.
Witsius probably did help popularize the expression, but does not seem to have taken credit for its origin. Furthermore, the 1626 date of Meldenius' publication is slightly before Witsius' birth, which naturally seals Witsius off from further consideration.
Melancthon (1497-1560) is old enough to antedate Meldenius' publication, and would even be old enough to cast Meldenius' originality into question, but it seems that the garden gate of Melancthon's garden may simply not date to Melancthon's time (which would hardly be surprising), and there appears to be no other record of Melancthon having heard of the saying.
At the end of the day, Meldenius has the edge on the others, given that his usage was the first to appear in print - that we have been able to recover (though I have not even been able to recover that). Here's an interesting brief discussion of Meldenius for those who may be interested (link). In short, L.P. Cruz appears to be justified in attributing the famous phrase to "Peter Medeirlin, a Lutheran pastor of the 17th Century." If you explore the final link above, you will find some reasonable speculation that Meldenius is a pseudonym (yes folks, people did publish pseudonymously before the Internet) for Medeirlin, based on a rearrangement of letters.
Regardless of who originated the saying let us follow the modified form published by J.A. Turretin, in which we maintain unity among Christians on the essentials (the gospel), liberty among Christians on the non-essentials (other doctrines), and both charity and prudence in all things.
P.S. Thanks to Albert for catching an error in the original version of this post.