To be of the flesh
is the same as behaving only in a human way
(behaving as someone who lacks the sanctifying influences of the Spirit). Paul gives two specific examples of such behaviour: jealousy and strife.
These examples are also included in the long list of works of the flesh
given in Galatians 5:19–21.
3 for you are still of the flesh. For while there is jealousy and strife among you, are you not of the flesh and behaving only in a human way?