Una Cum
This is a most important principle. Below are reproduced the Latin and an English translation from the Canon Law of the Mass celebrated by a priest, whose Pope is Michael and whose Ordinary is John Smith
Before going into the various rubrics and other times the Pope is mentioned in the liturgy, let us consider what is being said above. The priest says that he is in communion with the Pope Michael. What is he said he was in communion with Pope Benedict? Since Benedict XVI is a heretical usurper, he would be in communion with an Antipope, which is an act of schism. He is also acknowledge his obedience to the man he mentions as Pope and the man he mentions as his Bishop.. If he refuses obedience, to these men, then he is in schism with them.
A bishop inserts ut me indigno servo Tuo in the place of the mention of the local Ordinary. This means and me, Thy unworthy servant. The Pope omits both mentions and says instead: una cum me indigno servo Tuo, quem gregi tuo praesse voluisti. I your unworthy servant, of the flock you have firmly planted.
Good Friday:
Let us pray also for our most Holy Father Pope Michael, that our Lord and God, Who chose him for the order of the Episcopate, may keep him in health and safety for the good of His holy Church, to govern God's holy people.
The rubrics tell us tat during a vacancy of the Papacy, this prayer is omitted.
Litany of the Saints, which is sung on the Feast of Saint Mark, April 25th (Major Litanies) and on the Rogation days before the Feast of the Ascension (Minor litanies) Note all bound to recite the Divine Office are also bound to recite the Litany of the Saints on these days:
V/ Let us pray for our Sovereign Pontiff Michael.
R/ The Lord preserve him, and give him life, and make him blessed upon the earth, and deliver him not up to the will of his enemies.
And later on: Almighty, everlasting God, have mercy upon Thy servant, Michael, our Sovereign Pontiff, and direct him according to Thy clemency into the way of everlasting salvation, that by Thy grace he may desire those things that are pleasing to Thee, and perform them with all his strength.
What is to be said of the sede vacantists who omit mentioning the name of the Pope. By omitting any mention of a Pope are they therefore not schismatic? There actions are similar to the Orthodox and old-Catholics who also leave this completely out of their Masses. True, during a sede vacante this part is omitted, but a sede vacante is supposed to be of short duration. The sede vacantists conclude that Joseph Ratzinger and his predecessors Karol Wojtyla and John the Baptist Montini are antipopes, but claim that there is no way to end the vacancy. If they could point to a college of Cardinals convened to elect a Pope, who had yet to complete their task, then their claim would be valid. However, the logical conclusion of their claim is that THERE ARE NO CARDINALS, and thus the election of a Pope has devolved onto the Universal Church as taught by several theologians. By stating they do not know how to end the vacancy, they are stating that it cannot be ended, which is not only schismatic, but heretical denying the clear teaching of the only Vatican Council, that Peter will have perpetual successors in the Papacy, a teaching they supposedly professed to believe before their ordinations to Major Orders in taking the Oath Against the Errors of Modernism.