"On the same day our predecessor Clement VIII joyfully communicated the news of this happy event to the world by the Apostolic constitution Magnus Dominus et laudabilis nimis. How great was the joy and goodwill with which the Roman Church welcomed the Ruthenian people on their reception into the unity of the fold may be seen also from the Apostolic Letter Benedictus sit Pastor, issued on 7th February 1596, in which the supreme pontiff informed the metropolitan and the other Ruthenian bishops of the happy establishment of the union of their whole Church with the Apostolic See. In this letter the Roman pontiff briefly set forth what had been done in the matter at Rome; he gratefully extolled the work they had by God's mercy at length undertaken, and then decreed that the legitimate uses and rites of the Ruthenian Church could be preserved inviolate. "In the same manner as the council of Florence permitted, we too permit you to retain your rites and ceremonies, which in no way injure the integrity of the Catholic faith or our union." He goes on to say that he has asked the king of Poland to extend the protection of his patronage to the bishops and all appertaining to them, and also to pay them the fullest honor and, as they desired, to admit them to the senate of the kingdom. Finally he fraternally exhorts the bishops to meet as soon as possible in a full provincial council to ratify and confirm the union of the Ruthenians with the Catholic Church." - Orientales Omnes Ecclesias, encyclical of Pope Pius XII (1945).
Clement VII (January 30, 1592 - March 5, 1605) confirmed the Union of Brest with Magnus Dominus, and Pope Pius XII clearly concurred with that decision.
The Union of Brest only applies to Ruthenians and Slavs but within it, it clearly says:
"Since there is a quarrel between the Romans and Greeks about the procession of the Holy Spirit, which greatly impede unity really for no other reason than that we do not wish to understand one another—we ask that we should not be compelled to any other creed but that we should remain with that which was handed down to us in the Holy Scriptures, in the Gospel, and in the writings of the holy Greek Doctors, that is, that the Holy Spirit proceeds, not from two sources and not by a double procession, but from one origin, from the Father through the Son." - Union Of Brest signed by Metorpolitan Michael Rohoza of Kiev, Bishop Cyril Terletsky of Lutsk and Bishop Ostrih Pelchytsky of Pinsk (1595).
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"Credo unam, sanctam, catholicam et apostolicam Ecclesiam", the Church is one, that is vitally important. However within the church there can be different expressions of faith, and one must not necessarily agree with other rites on all things, but one must accept the other traditions as licit. The Filioque is a licit expression of Catholic faith, as is the Filioque-free creed. As both are Catholic expressions, it can not be said that one is right and the other is wrong. Pope Saint John Paul II famously said:
"Of course, in today's outlook it appears that true union is possible only in total respect for the other's dignity without claiming that the whole array of uses and customs in the Latin Church is more complete or better suited to showing the fullness of correct doctrine; and again, that this union must be preceded by an awareness of communion that permeates the whole Church and is not limited to an agreement among leaders." - Orientale Lumen (1995)
Hence respect is key in these matters, respect for both the Filioque and Filioque-free creed.