It hasn't never convinced me at all the idea that kata sarka here is referred to the verb "to know" and not to Christ.14 For the love of Christ controls us, because we have concluded this: that one has died for all, therefore all have died; 15 and he died for all, that those who live might no longer live for themselves but for him who for their sake died and was raised.
16 From now on, therefore, we regard no one according to the flesh. Even though we once regarded Christ according to the flesh, we regard him thus no longer. 17 Therefore, if anyone is in Christ, he is a new creation.The old has passed away; behold, the new has come.
From the other hand, it is quasi impossible that Paul knew a historical Jesus: his silence about a HJ in the epistles is very much sound beyond any imagination.
Therefore I am going to suspect the verse 16 as interpolation by a historicist Christian.
Without that verse 16, the conclusion in verse 17 follows more easily from verse 15.
The "therefore" of the verse 16 is totally without sense, at contrary of the "therefore" of the verse 17 (that is very much logical).
But why did the interpolator insert the verse 16?
To explain the silence of Paul about the HJ, giving the same reason given today by the likes of Ehrman: Paul wasn't interested about the HJ.
I suspect the interpolation as an anti-gnostic one. The gnostics considered any "psychic" people as mere people kata sarka. Only the "pneumatic" people were superior people. By remembering Jesus himself as one kata sarka, the catholic interpolator is exhorting the gnostic readers to revalue the flesh and also the hoi polloi, because Jesus himself was one of them.