First of all, nowhere does the Bible (God's word) say the catholic church is "the church", The Roman Emperor Constantine established himself as the head of the church around 313 A.D.. The first actual Pope in Rome was probably Leo I (440-461 A.D.), although some claim that Gregory I was the first (590-604 A.D.). This was long after Peter was dead. What the catholics believe about Peter being the first pope is not Biblical, it's what they made up to justify their existence.
The ecumenical movement is of the devil. The Bible clearly says that in the end, it will not be a huge bunch of followers saved, no, it will be a remnant, a few:
Revelation 12:17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.
Does the catholic church keep the commandments of God?
Exodus 20:8 “Remember the sabbath day, to keep it holy. 9 Six days you shall labor and do all your work, 10 but the seventh day is a sabbath of the Lord your God; in it you shall not do any work, you or your son or your daughter, your male or your female servant or your cattle or your sojourner who [e]stays with you. 11 For in six days the Lord made the heavens and the earth, the sea and all that is in them, and rested on the seventh day; therefore the Lord blessed the sabbath day and made it holy.
Oh, but do we need to still keep the commandments now that Jesus has risen?
Matthew 5:17 “Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. 18 “For truly I say to you, UNTIL HEAVEN AND EARTH PASS AWAY, NOT THE SMALLEST LETTER OR STROKE SHALL PASS FROM THE LAW until all is accomplished. 19 “Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven.
Has heaven and earth passed away?
Your ecumenical movement is under the assumption that Jesus wants all to unite, but Jesus said Matthew 10:34“Do not think that I came to bring peace on the earth; I did not come to bring peace, but a sword. 35“For I came to SET A MAN AGAINST HIS FATHER, AND A DAUGHTER AGAINST HER MOTHER, AND A DAUGHTER-IN-LAW AGAINST HER MOTHER-IN-LAW; 36and A MAN’S ENEMIES WILL BE THE MEMBERS OF HIS HOUSEHOLD.
People will not get along because of the disagreements in following God. I don't see anything like an ecumenical movement in the Bible.... Protestants left the church for valid reasons, the catholic church strayed from God's word, and nothing has changed. True followers of God will not join with them. It will be a remnant as the book of Revelation has stated.