The quest to understand and identify the opponents has left many an interpreter confused. The simple fact is that our data is incomplete, and we do not have a full profile of these "opponents". However, that does not mean we are completely ignorant about them, and today I'd like to propose that the opponents were Christians.
We may draw from several strands of evidence within the letters to come to this conclusion. Firstly, 1 Tim 1:6-7 speaks of those who have "deviated" [ἀστοχήσαντες] from the faith. Then, in 1:19-20 the author speaks of Hymenaeus and Alexander, as those who have “shipwrecked the faith.” This suggests that they had faith, or were faithful, but now this has been destroyed. The result of this shipwrecking of faith, is that they have been "delivered to satan so that they may learn not to blaspheme.”
The second letter to Timothy speaks in a similar fashion of those, namely Hymenaeus and Philetus, who have “swerved from the truth” [2:18] This suggests that they had the truth, but have now abandoned it in favour of a different view. In 3:8 the author uses Jannes and Jambres as an illustration of the opponents who have a "corrupt mind and counterfeit faith".
Thus, we may conclude that the opponents identified within the letters to Timothy were at one stage part of the Christian community in Ephesus, and were at one stage considered fellow Christians.