They are right, but not alone:

Michael Gerson used the Pope's recent visit to America as an opportunity to comment on what he calls the indispensibility of the Roman Catholic Church. http://www.washingtonpost.com/wp-dyn/content/article/2008/04/17/AR2008041703167.html
He gives two main reasons for this conclusion:

• First: ". . . the [RC] church is the main defender of reason in the modern world. It teaches the possibility that moral truth can be known through reflection and argument. It criticizes what Pope Benedict XVI has called the "dictatorship of relativism" -- a belief "that does not recognize anything as for certain and which has as its highest goal one's own ego and one's own desires." "Being an adult," says Benedict, 'means having a faith which does not follow the waves of today's fashions or the latest novelties.'"
• He also points out that the Roman Catholic Church is a defender of human dignity. Certainly, their record on the sanctity of life for the unborn and the aged bear witness to this.

I am glad that the Roman Catholic Church has helped to point in our world in the direction of sound thinking, and appropriate honoring of the value of life, but Gerson's notion that they are indispensible is not correct.

It is not my intention to debate the differences between Catholic and Evangelical teaching, but simply to point out that they exist and are significant. It will come as no surprise to you for me to tell you that I think I, and those who believe like me--Evangelicals--are right.

While we may lack the visibility of a Papal visit, Evangelicals likewise hold to the truth, we believe in human dignity, and we clearly proclaim the Evangel.

What is essential is not a particular denomination, but the proclamation of the truth of God's word in its fullest and most complete form.