Be pitiful, for every man is fighting a hard battle.

Ian MacLaren was the pseudonym or pen name of Rev. John Watson. Watson later discussed this theme in greater detail in a later book:

This man beside us also has a hard fight with an unfavouring world, with strong temptations, with doubts and fears, with wounds of the past which have skinned over, but which smart when they are touched. It is a fact, however surprising. And when this occurs to us we are moved to deal kindly with him, to bid him be of good cheer, to let him understand that we are also fighting a battle; we are bound not to irritate him, nor press hardly upon him nor help his lower self.

Variations:

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

Be kind. Everyone you meet is carrying a heavy burden.

Let us be kind, one to another, for most of us are fighting a hard battle.

1898 January 6, Congregationalist, In Brief, Page 9, Volume 83, Issue 1, Boston.