Even though we once regarded Christ according to the flesh, we regard him thus no longer (2 Cor 5:16).
What does Paul's cryptic comment mean? Here's a possibility: On the one hand there were thousands of Palestinian Jews, as well as however many Samaritans and gentiles, who heard Jesus preach, saw him work wonders and cast out demons. Yet many of them were his enemies. Indeed, some members of the Sanhedrin probably knew Jesus according to the flesh (in that sense), yet voted for his execution. Not to mention the lynch mobs who attempted to stone him or demanded that Pilate crucify him. So even firsthand knowledge of Jesus doesn't automatically save a person.
On the other hand, precious few Christians in churches planted by Paul had firsthand knowledge of Jesus. Living on mainland Greece, they didn't hear Jesus preach, see him work wonders and cast out demons. Yet they could be saved without knowing Jesus according to the flesh (in that sense).