Behold (βλεπετε). Same form for imperative present active plural and indicative. Either makes sense as in John 5:39 εραυνατε and 1 Corinthians 14:1πιστευετε.

Calling (κλησιν). The act of calling by God, based not on the external condition of those called (κλητο, verse 1 Corinthians 1:2), but on God's sovereign love. It is a clinching illustration of Paul's argument, an argumentum ad hominen.

How that (οτ). Explanatory apposition to κλησιν.

After the flesh (κατα σαρκα). According to the standards of the flesh and to be used not only with σοφο (wise, philosophers), but also δυνατο (men of dignity and power), ευγενεις (noble, high birth), the three claims to aristocracy (culture, power, birth).

Are called . Not in the Greek, but probably to be supplied from the idea in κλησιν.