No man can ever tell how far the blessing of his trivial acts of kindness, or other pieces of Christian conduct, may travel. They may benefit one in material fashion, but the fragrance may reach many others. Philemon little dreamed that his small charity to some suffering brother in Colossae would finds its way across the sea, and bring a waft of coolness and refreshing into the hot prison house. Neither Paul nor Philemon dreamed that, made immortal by the word of the former, the same transient act would find its way across the centuries, and would “smell sweet and blossom in the dust” today. Men know not who are their audiences, or who may be spectators of their works; for they are all bound so mystically and closely together, that none can tell how far the vibrations which he sets in motion will thrill. This is true about all deeds, good and bad, and invests them all with solemn importance. The arrow shot travels beyond the archer’s eye, and may wound where he knows not. The only thing certain about the deed once done is, that its irrevocable consequences will reach much farther than the doer dreamed, and that no limits can be set to the subtle influence which, for blessing or harm, it exerts.
Alexander Maclaren, The Epistles of St. Paul to the Colossians and Philemon, pp. 445-446

Truth. When I was a baby Christian, a woman who was guiding me was with me in a restaurant booth. She started telling me all the basics of becoming a Christian which I already knew, but kept my mouth shut out of respect. Later on she told me that the man sitting behind me was listening to every word she was saying and that her words while appearing to be directed at me, were actually for him. Lesson learned.