Ye did run well. — Again, as in Galatians 2:2, a metaphor from foot racing. The Galatians had made a good start, but suddenly changed their course.
Who did hinder you? — The metaphor here is not quite the same, but is somewhat akin to that just used. The original meaning of the word translated “hinder” is to “break up a road,” as an army before the advance of hostile forces.
The truth — i.e., the doctrine taught by St. Paul in opposition to the Judaising tenets which had been introduced into the Galatian Church.