If we read the book of Philippians very carefully, Paul seems to have had two groups of opponents.
In Philippians 1:17, Paul speaks of people preaching Christ out of envy and rivalry. It seems that these people in view are Christians, otherwise Paul would not rejoice in the fact that Christ is being preached. In light of this, we can say what Paul criticizes here is motivation. However, we can still say that they were Paul's opponents, to a certain degree.
However, when we move to Philippians 3, Paul speaks of another group. He calls them “dogs” (3:2) and “enemies of the cross” (3:18, 19). From the change in Paul’s tone, it appears that a heretical group is in view (at least from Paul's point of view). Most likely, this group emphasized the law at the expense of a teaching based on Paul's teachings on grace.
It is also possible there was another set of false teachers, who emphasized a theology of “perfection,” or to put it in theological terms, an undue emphasis on overrealized eschatology. Perhaps, this is why Paul emphasizes that he has not been made perfect yet in that same chapter.