Often, throughout history, the Roman Catholic Church (RCC) has asserted its primacy among the Christian faiths of the world -- with some justification. The historical church began in Jerusalem with James the brother of Jesus as the leader of the church (cf. Acts 12.2 and Galatians 1.19 & 2.7-9). Tradition suggests Peter went to Rome to begin a church there, though there is absolutely no biblical evidence for this. In any event after 70, when Jerusalem was destroyed by the Romans, the church in Jerusalem was scattered and the center for Christianity was relocated to Rome (note, however, that by 70 Peter had been dead over three years).
During the ensuing years the Church in Rome became more and more powerful as it defined what orthodox Christianity was and how the church should respond to alternate theologies. Time and again the church asserted its muscle through church councils and treatises to insure doctrinal purity. As it did so, other centers of Christianity contended for autonomy, but the Roman Church repeatedly quelled these rebellions. Eventually, a strong faction in Constantinople claimed they were the one, true church and Rome and the Eastern Orthodox Church parted ways, both claiming to have the corner on orthodoxy.
For centuries, indeed for nearly two millennia, the RCC continued to claim to be the sole champion for the Christian faith and orthodox truth. Meanwhile, independent thinking individuals and congregations denied their claim. Yet, other than the split between the Roman Church and the Eastern Orthodox Church, the church remained essentially unified for 1,500 years. Indeed, in that time period the RCC was the one, true church because for all practical purposes, there was no other church. However, Martin Luther, John Calvin, and Ulrich Zwingli changed all that by launching the Reformation, wherein they lead a protest against the RCC (hence the term Protestant -- from protest). The claim of the RCC as the one, true church has been seriously challenged ever since.
Nonetheless, the Roman Catholic Church continued to maintain it was the only provider and protector of the faith and attested this by refusing to participate, or allowing its parishioners to participate, in any activities sponsored by other churches.
But all that changed between 1962-1965. In these years the RCC held what is widely known as Vatican II. This council changed the face of the RCC and its attitudes towards churches outside the Roman Catholic faith. In its Decree on Ecumenism and the Declaration on Religious Freedom the RCC softened its view on being the single holder of truth and suggested it is the duty of all to seek "truth." Indeed, the documents imply that the RCC is not the lone provider of truth and that God-only is sole provider and protector of truth.
The reality is this: Truth is bigger than any one individual, congregation, denomination, church, and even bigger than any one religion. God alone has the corner on truth; the rest of us just have whatever God has chosen to reveal -- whether through creation (Romans 1.19-20), through the prophets (John 12.38), through Jesus, or through whomever and whatever God chooses.
The fact is, there is really only one true church that carries with it the fullness of God's love, God's grace, and God's truth -- and that's the one with its address in the heavenly courts. All the rest of us are struggling to live and teach the truth we've discovered.