Jesus Christ, the second person of the Blessed Trinity, founded one Church, and one Church only. The members of that Church are called Christians.
In the 11th century, some people broke away from the Christian Church. They are rightly called schismatics.
In the 16th century, new groups of heretics and schismatics flourished. By the present day, these schismatics have managed the neat trick of persuading everyone to call them Christians, even though they have not renounced their heresies nor rejoined the one Christian Church founded by Christ.
Actual Christians have gotten so used to calling these schismatics Christians that they have started to mistake them for actual Christians. (Given how off-the-wall the pagans and heathens have become in comparison, the confusion is understandable.)
Within the last century, both actual Christians and these schismatics (whose various faiths number in the tens of thousands) sat around and said “we all follow Christ. It is a scandal that we Christians are not all united. Let us set aside our differences, and come together in unity.”
It is a nice thought, but it is not Christian unity.
Indeed, it is a scandal that those who wish to follow Jesus Christ are not united. Since there is only one Christian Church, the only way to end that scandal and enjoy Christian unity is to convert and become a member of that Church.
This means becoming a member of the Catholic Church. Full stop.
There is no other way for aspiring Christians to enjoy Christian unity. There can be no unity without uniformity of faith. Ephesians 4:5 puts it bluntly: “One Lord, one faith, one baptism.” The minute one deviates from the one Catholic faith, one’s claim to the title “Christian” is in jeopardy. But a willingness to be corrected and submit to the Church greatly mitigates the danger.
The pope, the vicar of Christ on earth, is necessary to establish and preserve unity in the Church. As St. Jerome said (as quoted in the Catechism of the Council of Trent):
“Following no chief but Christ, I am united in communion with your Holiness, that is, with the chair of Peter. I know that on that rock is built the Church. Whoever will eat the lamb outside this house is profane: whoever is not in the ark of Noah shall perish in the flood.”
If you are reading these words and are not Catholic, I beg you: convert to the one Church founded by Jesus Christ. I do not want you to perish in the flood. How your immortal soul spends eternity is at stake.
If you have any questions, email firstname.lastname@example.org and I will do my best to answer.