Definition (house-style)
Theopaschism is the view that God the Father — not merely the human nature of the Son — suffered in and through the cross of Christ. The term originates in early Christian debates of the fifth and sixth centuries, where the theopaschite formula “one of the Trinity suffered” was used to affirm that the suffering at the cross was genuinely divine suffering. Theopaschism is distinct from patripassianism: patripassianism denies the Father-Son distinction (the Father suffered as the Father, without the Son being a separate Person), whereas theopaschism denotes a co-suffering of the Father in and with the Son — the Father as a recognizably distinct Person suffering through his indwelling in the Son.
Author variants
Warnock
Warnock articulates a radical participation of the Father in the Son’s suffering at the cross. The Father indwells the Son; when the Son suffers, the indwelling Father suffers with him:
“When He hung on the cross… it was not so that God the Father was indifferent toward the cries of His Son going through that unspeakable suffering… but in the very real sense God the Father Himself felt the pain of every nail that was driven into His hand, and every thorn that pierced His brow.”
[Warnock, The Hyssop that Springeth Out of the Wall, hyssop2.html]
Warnock simultaneously affirms the mystery of the triune structure and explicitly denies that the Father and Son are identical. His formulation approaches patripassianism but formally maintains the Father-Son distinction: the Father is the indwelling God in the Son, experiencing the cross from within. This is functional theopaschism — the suffering of the Son is simultaneously the suffering of the indwelling Father. The theological tension is that Warnock elsewhere insists on preserving the Father-Son distinction (“it disturbs me that so many Christians would regard God the Father as one Person… and God the Son as another”), yet his theopaschite formulations put that distinction under pressure.