I sorta agree, but also sorta disagree with the above. In presenting the "truth" to people, I think it's important to realize that we are not the voice of God. We can say the same words to unbelievers as Jesus said to the Pharisees, and think we're doing "right" because we're quoting right out of the Bible. But most of us don't have the innate authority that Jesus did that commanded the attention of people. Jesus could be "in their faces" on "on their cases" with total knowledge that He was doing the right thing. That's not so true with the rest of us.
The example in the OP was given about watching mines being placed on a pathway. Do you warn people? Of course. But many won't believe you. They need "proof" — that proof can be provided by you, or they can test it themselves.
How would you show them? I would throw rocks or some other heavy object that I could find in order to set off one of the explosives. Nothing like examples to prove one's point.
In the same manner, personal testimony is one of the most powerful weapons against Satan.
Just my 2¢