You obviously don't know what you are talking about and you obviously don't understand what Pope Pius XI is saying in Mortalium Animos. Maybe you have never actually read the encyclical, or if you did, you don't understand it.
Mortalium Animos is a condemnation of these ecumenical meetings among "pan Christians" or pseudo Christian groups of non-Catholic "Christians" and Catholics with the object of setting aside their differences and forming some sort of an amalgamation of common beliefs.
Moritalium Animos is an encyclical issued against the false attempts at religious unity, today collectively known as “ecumenism”, that were beginning to blossom at the time.
"..it is clear that the Apostolic See cannot on any terms take part in their assemblies, [ecumenical meetings with non-Catholics] nor is it anyway lawful for Catholics either to support or to work for such enterprises; for if they do so they will be giving countenance to a false Christianity, quite alien to the one Church of Christ.
Who then can conceive a Christian Federation, the members of which retain each his own opinions and private judgment, even in matters which concern the object of faith, even though they be repugnant to the opinions of the rest? And in what manner, We ask, can men who follow contrary opinions, belong to one and the same Federation of the faithful? For example, those who affirm, and those who deny that sacred Tradition is a true fount of divine Revelation; those who hold that an ecclesiastical hierarchy, made up of bishops, priests and ministers, has been divinely constituted, and those who assert that it has been brought in little by little in accordance with the conditions of the time; those who adore Christ really present in the Most Holy Eucharist through that marvelous conversion of the bread and wine, which is called transubstantiation, and those who affirm that Christ is present only by faith or by the signification and virtue of the Sacrament; those who in the Eucharist recognize the nature both of a sacrament and of a sacrifice, and those who say that it is nothing more than the memorial or commemoration of the Lord’s Supper; those who believe it to be good and useful to invoke by prayer the Saints reigning with Christ, especially Mary the Mother of God, and to venerate their images, and those who urge that such a veneration is not to be made use of, for it is contrary to the honor due to Jesus Christ, “the one mediator of God and men.
So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it...To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it. During the lapse of centuries, the mystical Spouse of Christ has never been contaminated, nor can she ever in the future be contaminated,
Furthermore, in this one Church of Christ no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors...The Catholic Church is alone in keeping the true worship. This is the fount of truth, this the house of Faith, this the temple of God: if any man enter not here, or if any man go forth from it, he is a stranger to the hope of life and salvation. .
Let, therefore, the separated children draw nigh to the Apostolic See, set up in the City which Peter and Paul, the Princes of the Apostles, consecrated by their blood; to that See, We repeat, which is “the root and womb whence the Church of God springs/
We humbly beg that He would deign to recall all who stray to the unity of the Church! In this most important undertaking We ask and wish that others should ask the prayers of Blessed Mary the Virgin, Mother of divine grace, victorious over all heresies and Help of Christians, that She may implore for Us the speedy coming of the much hoped-for day, when all men shall hear the voice of Her divine Son, and shall be “careful to keep the unity of the Spirit in the bond of peace.
You, Venerable Brethren, understand how much this question is in Our mind, and We desire that Our children should also know, not only those who belong to the Catholic community, but also those who are separated from Us: if these latter humbly beg light from heaven, there is no doubt but that they will recognize the one true Church of Jesus Christ and will, at last, enter it, being united with us in perfect charity. "
TRANSUBSTANTIATION means a change of substance. The Roman Catholic Church teaches that the whole substance of bread and wine is changed into the physical body and blood of Jesus Christ. This RCC teaching is based on the words of Jesus in Matthew 26: 26-28: "This is my body, this is My blood." The RCC teaches that these words are to be taken completely literalistically. The RCC believes that in His command "Do this in remembrance of Me", Jesus was authorizing and empowering the apostles as priests to carry on this practice of transubstantiation, that they might offer Christ to the people to be literally eaten and also to be continually sacrificed for their sins.