...is the New Testament.
All Christians believe that the Holy Spirit guided the Church over centuries to determine which books do and do not belong in the New Testament. This decision is regarded by all Christians as infallible (protected from error). Catholics believe that this same protection extends to all of the Church’s doctrines that have developed over the centuries.
As noted in several previous posts, it took hundreds of years for the Church to get to a final list of books in the New Testament. All Christians today recognize that it was the Holy Spirit that guided the Church to its final correct conclusion. Considering that neither the Bible nor the teaching of the apostles suggests that this process should change, the Catholic Church trusts that the Holy Spirit is continuing to protect the Church from error in doctrine.
Catholic doctrines generally develop over time. Some doctrines about Mary, for example, were officially pronounced very recently. However, Catholics consider these infallible because, just like with the New Testament, the teachings of Mary were widely held very early on, were refined and more fully understood over time, and have been found to be useful in growing closer to Jesus. The reason for our confidence in the New Testament and in Mary's assumption into heaven is the same! If we trust one, we have no reason to doubt the other.
It would be very odd indeed to be confident that the Holy Spirit would guide the Church to discover what books belong in the New Testament and to then deny that the Holy Spirit could guide the Church into other truths. It would also be odd to say the doctrine cannot develop or be more fully understood over time when that's precisely the process by which we cane to the 27 books we revere in the New Testament today. In may ways, the process of infallibly determining the books of the New Testament is a template for how the Holy Spirit has guided the Church into truth over 2,000 years and continues to do so today.