{To number or compare ourselves} (enkrinai (8820)sunkrinai).
Paronomasia here, play on the two words. Enkrinai is first
aorist active infinitive of old verb, but here only in N.T., to
judge among, to judge one as worthy to be numbered among as here.
The second verb sunkrinai (first aorist active infinitive of
sunkrin(935c), old verb, in N.T. only here and  1Co 2:13 )
originally meant to combine as in  1Co 2:13  (which see), but
here it has the sense of "compare" not found in the old Greek.
The papyri use it to mean to decide. Plummer suggests "to pair
and compare" for the play on the words here. {Measuring
themselves by themselves} (en heautois heautous metrountes). Or
"in themselves." Keenest sarcasm. Setting themselves up as the
standards of orthodoxy these Judaizers always measure up to the
standard while Paul falls short. {Comparing themselves with
themselves} (sunkrinontes heautous heautois). Associate
instrumental case heautois after sunkrinontes (verb just
explained). Paul is not keen to fall into the trap set for him.
{Are without understanding} (ou suni(8373)in). The regular form for
present active indicative third plural of suni(886d)i, to
comprehend, to grasp. Some MSS. have the late form suniousin
(omega form suni(935c)). It is a hard thing to see, but it is true.
These men do not see their own picture so obvious to others ( Eph
5:17  1Ti 1:7 ). Cf.  Mr 8:17 .