As Paul goes around preaching the gospel, a slave to the victorious Jesus, an aroma goes up. The spreading of the fragrance of the knowledge of God is in view. Paul returns to this in 4v6, a knowledge that isn't facts and information but the glory of the gospel in the face of Jesus. The aroma in view is firstly to God, as a pleasing sacrifice (Leviticus?). Paul's evangelism as he declares the gospel is worship. And then it's second direction is toward people. Some are the perishing, those who (4v3-4) are blinded by the god of this age, and are called unbelievers. Others however are the saved, for whom the veil is removed (3v16-18).
Calvin captures the picture:
"God, through his instrumentality, wrought powerfully and gloriously, perfuming the world with the health-giving odor of his grace, while, by means of his doctrine, he brought some to the knowledge of Christ. He carries out, however, the metaphor of odor, by which he expresses both the delectable sweetness of the gospel, and its power and efficacy for inspiring life. In the mean time, Paul instructs them, that his preaching is so far from being savourless, that it quickens souls by its very odor. Let us, however, learn from this, that those alone make right proficiency in the gospel, who, by the sweet fragrance of Christ, are stirred up to desire him, so as to bid farewell to the allurements of the world."Paul preaches the gospel. He went to Troas for it, though quickly moved on despite there being good opportunities for it, because he needed to see Titus. The preaching of the gospel is an aroma to God, a pleasing act of worship to him. But it's also a smell among people that divides everyone - some to life, some to death.
Calvin, with some more tasty language to talk about the gospel:
"wherever there is a pure and unvarnished proclamation of the gospel, there will be found there the influence of that odor... Such is the influence of the Gospel in both respects, that it either quickens or kills, not merely by its taste, but by its very smell. Whatever it may be, it is never preached in vain, but has invariably an effect, either for life, or for death.... The Gospel is preached for salvation: this is what properly belongs to it; but believers alone are partakers of that salvation. In the mean time, its being an occasion of condemnation to unbelievers. Christ came not into the world to condemn the world, for what need was there of this, inasmuch as without him we are all condemned? — that arises from their own fault.... We must always, therefore, distinguish between the proper and natural office of the Gospel, and the accidental one (so to speak) which must be imputed to the depravity of mankind, to which it is owing, that life to them is turned into death."Piper on things that are heart-breaking and heart-rejoicing:
"some people smell the sacrificial love of Christ in the life of a missionary and it only smells like death. They hear the gospel and all they hear is death. They look at the cross and all they see is death. They see no life. No hope. No future. No joy. And so they turn away. And if they turn away forever, they die. They are the perishing. The smell of death leads to death. That’s the heart-breaking side of missions..... But there is the heart-rejoicing side of missions ...to one a fragrance from death to death, to the other a fragrance from life to life.” Those who are being saved smell the death of Christ as the aroma of life. They see in his death the substitute that they so desperately need before God. The Son of God dying in their place is the fragrance of life. So they don’t turn away. They believe him and receive him and embrace him and treasure him and they live—forever. Smelling Christ as the aroma of life gives life."