Ecuмenism, properly understood, is nothing more or less than the effort to bring all separated communities into corporate reunion with the Catholic Church. When there are prospects for the return of some 300 million souls, as among the Eastern schismatics, then an ecuмenism of return is a worthy prospect, and, if properly pursued is likely to be more fruitful than individual conversions alone. If the leaders of the Church act for zeal of souls, they could achieve great good. If they act out of human respect, they stand condemned in that matter, as St.Peter, Prince of the Apostles, was.
Now a cleric can resist such a Pope, to his face, as St.Paul did. But I do not think that applies straightaway to the laity.
Thus, St.Catherine,
"Even if the Pope were Satan incarnate, we ought not to raise up our heads against him, but calmly lie down to rest on his bosom. He who rebels against our Father is condemned to death, for that which we do to him we do to Christ: we honor Christ if we honor the Pope; we dishonor Christ if we dishonor the Pope."