First, let me offer a disclaimer. I am very much in favor of ecumenism and interfaith dialogue. The Church asks us to honor the faith of other people and not proselytize those who do not wish to have the Church's complete truth proclaimed to them.
With that said, though, I do believe there is a "true" ecumenism and a "false" ecumenism. The true ecumenism of the Catholic Church recognizes that the Church that Jesus Christ founded is the Catholic Church headed by the Christ's visible vicar on earth, the successor to St. Peter, the Pope. As such, the Catholic Church has the fullness of truth. The Catholic Church also safeguards this truth, what is called by many as the "Deposit of Faith." As such, we see all validly baptized Christians as members of the One, Holy Catholic and Apostolic Church. Depending on their status in the line of the denominations that evolved after the Protestant Reformation, they are more or less in communion with us, but not in full communion. Again their status depends on how much of the Deposit of Faith they accept and the degree to which their sacraments are either valid or invalid according to the mind of the Magisterium.
False ecumenism believes that all Christian denominations are more or less equal.Many would even refer to the Catholic Church as one among many different denominations. It really doesn't matter which one you belong to as long as you believe in Jesus and try to follow Him as your conscience directs you. This is especially tempting for Catholics who are the minority in the communities in which they live or are in "ecumenical marriages." We don't want to offend the religious sensibilities of those who are close to us, so we tend to buy into a false egalitarianism.
We must be clear, though, that the Catholic Church is precisely that the Church. Eastern Orthodoxy can also be viewed as a Church not a denomination. Protestants, though are denominational. In the technical sense they can only call themselves the "Church" only in reference to the Church they separated from, the Catholic Church.
Three areas that many Catholics today are susceptible are "the cult of the personality" of the priest or minister and the power of inspirational preaching and the strong bonds of fellowship that many Protestant denominations are quite good at developing. If a preacher has a pleasant and winning personality, knows how to speak from his heart and to touch the hearts of those who listen to him combined with a powerful sense of friendship and fellowship among fellow worshipers, Catholics may think that their needs are being fulfilled. This is especially true if their own Catholic parish lacks a charismatic priest, who is deficient in preaching and personality and the congregation is cold and unfriendly. Why not then join a denomination that satisfies?
The three main reason not to leave the full communion of the Catholic Church that are relevant here:
1. The Catholic Church was founded by Jesus Christ and He said the gates of hell would not prevail against her. Why in the world would a Christian intentionally join another denomination founded by a mere mortal, like Martin Luther, John Calvin or the vast variety of non-denominational congregations founded by individuals who turn it into a corporation in which they are the CEO and power is handed down to family members? It is a family business in other words.
2. The validity of our Sacraments. For most Protestant denominations, they have only two valid sacraments, Holy Baptism and in some cases Holy Matrimony. Since the Sacrament of Baptism is essential for Church membership and thus salvation the Catholic Church has always taught that in an emergency a lay person can baptize if they use water and the correct formula, "I baptize you in the Name of the Father and of the Son and of the Holy Spirit. Thus the Catholic Church recognizes the baptism of Protestants if done according the criteria above. Since marriage is a divine right if there are no impediments, the marriages of two baptized Christians who are Protestant is a valid sacramental marriage as it is not dependent on the one who witness the marriage for its sacramentality. However, with few exceptions, Protestants do not have any other valid sacraments including first of all Holy Orders (deacons, priests and bishops) the latter two necessary for the valid celebration of the other sacraments including the Mass or the Most Holy Eucharist, not to mention Confirmation, Penance, Anointing of the Sick and Holy Orders. Why would anyone give up the Sacramental system of the Church that Jesus Christ Himself instituted for a simulated sacrament or no sacraments at all? Leaving the full communion of the Church means no Holy Communion and if it is celebrated by the Protestant denomination it is only a symbol or simulated sacrament. It is not valid.
Thirdly, if one departs the practice of the Catholic Church for a denomination that is not in full communion with the Catholic Church, one must also divest themselves of many other teachings such as the role of the Blessed Virgin Mary, the Communion of saints, Purgatory and the finer nuances concerning dogma and doctrine.
Who wants incomplete Christianity when the complete package is available in the Roman Catholic Church? Only those who do not think there is any difference amongst the various Protestant denominations and that all religions are equal.
True ecumenism does recognize that God's grace is present in these Protestant denominations and that what is lacking in them can be made up by God and His grace. Certainly good preaching, a love for the Scriptures, lives built upon the moral principals of the Bible are all signs of God's grace actively bestowed upon the well-intentioned and good of heart. As well friendly congregations, great fellowship and social activism are signs of God's grace freely given to these denominations that at times puts us Catholics to shame. So we do not disparage the gracious love that God bestows upon those denominations. While deficient in so many essentials of the Church, God nonetheless enables them to experience an abundance of good will and love.Where ever two or three are gathered in the Lord's name, Jesus is there! In terms of ecumenical dialogue, we can learn much from their fine preaching, their fellowship and working with them in serving the needs of the poor rather than competing in this arena.