“Be kind, for everyone you meet is fighting a hard battle.”

Attributed to Philo in How Do We Know When It's God?: A Spiritual Memoir (1999) by Dan Wakefield. It has also been wrongly attributed to Plato and Ephrem the Syrian. It is a variant of the Christmas message "Be pitiful, for every man is fighting a hard battle," written by the Scottish preacher Ian Maclaren (also known as John Watson) in 1897.
Be Kind; Everyone You Meet is Fighting a Hard Battle. Plato? Philo of Alexandria? http://quoteinvestigator.com/2010/06/29/be-kind/.
Misattributed

Source: The Candymakers

Source: Styxx

Source: What Happened to Lani Garver

Speaking Of Love (1980)
Variant: The hardest battle you are ever going to have to fight is the battle to be just you.

Source: Bleach, Volume 01

Et scahiez que Anglois et Escoçoiz, quant ilz se treuvent en bataille ensamble, sont dures gens et de longue alainne, et point ne s'esparngnent, mais s'entendent de eulx mettre à oultranche, comment qu'il prende. Ilz ne ressamblent pas les Alemans qui font une empainte, et, quant ilz voient qu'ilz ne puellent rompre ne entrer en leurs ennemis, ilz s'en retournent tout à ung fais.
Book 3, p. 345.
Chroniques (1369–1400)

Source: The Sweet Far Thing