Jamieson, Fausset and Brown on Galatians 2:4

4. And that--that is, What I did concerning Titus (namely, by not permitting him to be circumcised) was not from contempt of circumcision, but "on account of the false brethren" (Ac 15:1, 24) who, had I yielded to the demand for his being circumcised, would have perverted the case into a proof that I deemed circumcision necessary.

unawares--"in an underhand manner brought in."

privily--stealthily.

to spy out--as foes in the guise of friends, wishing to destroy and rob us of

our liberty--from the yoke of the ceremonial law. If they had found that we circumcised Titus through fear of the apostles, they would have made that a ground for insisting on imposing the legal yoke on the Gentiles.

bring us into bondage--The Greek future implies the certainty and continuance of the bondage as the result.