Back before Vatican II, Catholics were warned to keep clear of Protestantism. It was a sin to attend a service in a Protestant church, for example, and I recall horrendous stories of Catholics who could not attend the baptisms, confirmations, weddings and funerals of Protestant kinfolk if such services were held in a Protestant church. Luther was depicted as a thoroughly evil heretic who was condemned to hell for breaking his ordination vows, even marrying a former nun, etc.
Then came Vatican II and John XXIII (my favorite Pope). Almost overnight the attitude of Catholicism changed. Ecumenical activities were encouraged. Ecumenical services were commonplace. Priests joined what had formerly been ministerial associations. Some priests even co-officiated at weddings, sometimes if held in Protestant churches. The Protestants were no longer condemned as heretics. Suddenly they were 'separated brethren'. They were fellow Christians without the fullness of truth, mostly because they were raised as Protestants and usually not because of their own fault.
Unfortunately - in my view - speaking honestly - there still are Catholics who hold to the pre-Vatican view. They speak pompously and piously of being part of the 'one true church'. I watch EWTN regularly, and that narrow viewpoint often comes through, on such programs as "Journey Home" and others. To me, this contradicts the spirit of Vatican II. It is one thing to believe that Catholicism is the one true religion. It is quite another to diminish the sincere faith of Lutherans or Methodists, Baptists or Presbyterians.
Over the years I have reached this conscensus - personally. God and the mysteries of the universe are beyond human understanding. We "know in part" (I Cor. 13), while the core of the faith is love (again, I Cor. 13). All Christians who genuinely seek to follow Christ and his call to 'love one another' are my brothers and sisters, whatever their church connection. And I would go further. Jews, Muslims and others are my brothers and sisters. too, if they live by that principle of love. I spent awhile living with a Hindu family in India years ago, and if they don't get to heaven I doubt if you and I do. I don't believe God looks at our creeds or church affiliation. God looks at our hearts. I believe it was John Wesley who said in effect: "If you love God as I do, if you wish to serve God as I do, let us join hands and walk together...."
That is the sum and substance of the gospel and all the doctrinal details and ceremonial rites matter very little when compared to that central message of love.
A Jehovah Witness came to my door this morning. A fine young woman I'm sure. They too believe they alone have the one true faith, as do Mormons, many Muslims, and we could go on. The true faith, as expressed by Jesus, is summed up in the Sermon on the Mount. Sadly, too many Christians claim to preach love, humility, unity and peace while they actually promote attitudes that encourage bigotry, arrogance, separation, and antipathy.
Most Catholics I know agree with me. And ditto for most Protestants I know, too. God bless them all. God bless the whole world and all who live in it, regardless of their religious views or connections.