How is Paul’s prayer for Onesiphorus connected to arguments for purgatory?

In Roman Catholic tradition, the practice of praying for the dead is considered a key biblical warrant for the doctrine of purgatory. Advocates of the doctrine cite the Apostle Paul’s prayer for Onesiphorus in 2 Timothy 1:18 as evidence that such intercession is both valid and meaningful after death.1

The argument posits that if Onesiphorus were already in a state of final reward or eternal damnation, prayer on his behalf would be unnecessary. Therefore, the act of praying for him implies the existence of an interim state—a place of purification or "mixed" status—where the deceased may benefit from the prayers of the living before the Day of Judgment. This interpretation aligns with the broader Catholic view that while salvation is settled at death, those destined for heaven may undergo a period of correction or cleansing.21