Albert Barnes Commentary 2 Timothy 2:18

"men who concerning the truth have erred, saying that the resurrection is past already, and overthrow the faith of some." — 2 Timothy 2:18 (ASV)

Who concerning the truth have erred. To what extent they had erred is unknown. Paul mentions only one point—that pertaining to the resurrection; but says that this was like a gangrene. It would certainly, unless checked, destroy all the other doctrines of religion. No one can safely hold a single error, any more than they can safely have one part of their body in a state of mortification.

Saying, that the resurrection is past already. It is not known in what form they held this opinion. It may have been, as Augustine supposes, that they taught that there was no resurrection but that which occurs in the soul when it is recovered from the death of sin and made to live anew. Or it may be that they held that those who had died had experienced all the resurrection they would ever have, by passing into another state and receiving at death a spiritual body fitted to their mode of being in the heavenly world. Whatever was the form of the opinion, the apostle regarded it as a most dangerous error, for correct views of the resurrection undoubtedly lie at the foundation of a proper understanding of the Christian system. (Compare to 1 Corinthians 15:12 and following).

And overthrow the faith of some. That is, on this point, and, it would seem, on all the related subjects of Christian belief. (Compare to 1 Timothy 1:19–20).