Unknown, and yet well known: the sense may be that they are unknown in the sense that most people do not know them, or, more specifically, the sense is probably that their apostolic authority is “unrecognized” by some Christians (so Barrett). The Greek does not state who knows Paul and his associates. According to Good News Translation it is other Christians (“by all”). Revised English Bible says “whom all men know,” and Contemporary English Version says “but well known to you.” Possibly Paul means that, even though certain Christians do not recognize his authority, yet God does, that is, “we are known by God.” Or perhaps Paul is thinking of both people and God. The passive ideas will have to be made active in many languages. Some suggested models are: “people do not accept us, but God accepts us” or, following the other interpretation, “people say they don’t know us, but they are always watching us.” Barclay reads “no one knows us and everyone knows us.” However, it is probably more likely that different agents are intended for the words “known” and “unknown.” The probable meaning is “some Christians do not recognize our apostolic authority, yet our authority as apostles is well known to Christians everywhere.”
As dying, and behold we live: Paul was constantly in danger of losing his life, and that seems to be the sense of the words as dying. On the word behold see 5.17 and 6.2. A possible model for this part of the verse: “they say that we are dying, but look! we are still alive.” Or “people think we are dying, yet we live on!”
As punished, and yet not killed: punished by whom? The parallel of Psa 118.17-18 to 6.9bc suggests that God is the implicit subject of the verb punished. In agreement with the Old Testament teaching that God’s punishment is intended to lead to correction (see, for example, Psa 94.12; 119.67), Paul may have interpreted his sufferings as God’s disciplinary action. On the other hand, the events of Paul’s life as we know them from Acts and from his own letters give support to the view that Paul has in mind here punishment by other human beings. In languages without appropriate passive forms, one may wish to say “they [indefinite] persecute us, but they do not kill us” or “we suffer greatly, but we do not die.”
Quoted with permission from Omanson, Roger L. and Ellingworth, Paul. A Handbook on Paul’s Second Letter to the Corinthians. (UBS Handbook Series). New York: UBS, 1993. For this and other handbooks for translators see here .