Source, in full
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.
Notes from Wikiquote
Be Kind; Everyone You Meet is Fighting a Hard Battle. Plato? Philo of Alexandria?.