For we are an aroma of Christ to God among those who are being saved and those who are perishing; to the one a fragrance from death to death, to the other a fragrance of life to life. Who is sufficient for these things? - 2 Corinthians 2.15-16

Elephants in Kenya can tell a Masai tribesman from a Kamba by a single sniff from a considerable distance. That's good for the elephant, because the Masai hunt elephants and the Kamba don't. 

It's good to know in advance whether someone is on your side. Of course, that doesn't help much if you can't get away and don't have the option of changing sides.

Paul tells the Corinthians that Christians give off a different spiritual smell depending on whose doing the huffing. He's working out a rather complex military metaphor that he introduces in verse 14: the Roman military parade. When a Roman general won a great victory, the senate voted him a triumph. They rounded up all the captured booty along with a selection of the prisoners and marched the whole cavalcade down the main street of town where, along with other ceremonies, they sacrificed the POWs to the appropriate god. One feature of these festivities was the burning of clouds of incense. 

If you were a member of the conquering army or citizenry, it probably smelled great. If you were a prisoner chained to a chariot wheel, not so much. Christ moves in often-unseen victory through our world and every person can pick a side. One day, however, when the battle ends and we behold the Lord in his glory, that choice will be fixed forever.

Who is sufficient for these things? Paul feels the weight of the burden: When we declare the gospel, we force people to pick a side, and they define their eternal destiny when they turn up their noses at the scent of grace. Our problem, in other words, is not whether people like our smell; it is whether we smell of salvation.