In 1378, there was a concrete case of “an ordinary lay Catholic in the pews” who had to decide whether Clement VII was Pope because God “allowed the entire College of Cardinals” to recognize him as Successor of Peter instead of Urban VI (still very much alive and kicking).
Against the view of Bishop Schneider that: “There is no authority to declare or consider an elected and generally accepted Pope as an invalid Pope,” Doctor of the Church, St. Catherine of Siena did exactly that: declared and considered an elected and generally accepted “Pope” Clement VII an antipope and defied the entire college of cardinals when she wrote to them:
…I tell you that you [cardinals] did wrong, with the antipope… he was chosen a member of the devil… you have committed all these faults in regard to this devil… to confess him as Pope, which he surely is not…(emphasis added).[1]
Wener and Vidal also disagree with Fr. Harrison about whether individuals are allowed to believe that a putative Pope is an antipope:
…they cannot be numbered among the schismatics, who refuse to obey the Roman Pontiff because they consider [him]… suspect or doubtfully elected on account of rumors in circulation.[7]
https://onepeterfive.com/is-francis-pope-my-reply-to-fr-brian-harrison/