“Sometimes Christians comment about the need to be saved on unrelated YouTube videos. These comments get many thumbs up, but also make many people angry. Are they useful ways to witness?”
Each block near where I work has two or three parking meters. On the one I use most frequently someone has scratched “You need Jesus. Pray every day.” I have a friend who makes it his habit to attend protests, rallies, parades and major gatherings all over the city carrying a sign with a Bible verse on it. Then there is the classic “John 3:16” meme that has appeared for years in various forms at televised sporting events.
Like those YouTube comments from Christians that show up out of the blue where nobody is asking for them, these are all unsolicited expressions of faith in unexpected places intended to make people think about eternity.