Hello Matt and Ahimsaman,
Catholics believes that the Catholic Church has continued the belief and practice of the apostles. For example I believe that our belief in the true Body of Jesus and communion and our practice of mass are learned from the apostolic Church.
2 Timothy 3:14 But you, remain faithful to what you have learned and believed, because you know from whom you learned it,
1 Corinthians 11:2 I praise you because you remember me in everything and hold fast to the traditions, just as I handed them on to you.
2 Thessalonians 2:15 Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours.
There is no question that Scripture makes it clear that there are teachings outside of Scripture. Therefore Scripture makes it clear that it does not contain all the truths of faith. Yes, St Paul says that Scripture equips one for every good work. Showing from Scripture that Scripture itself does not contain all the teachings is a good work.
Therefore, one can never use Scripture alone to understand the fullness of the Christian faith. This is made clear from Scripture itself.
So for a person to say that their Church teaches the truth and they base all their beliefs and practice only on Scripture is clearly wrong.
As far as what these traditions, letters, and oral teachings are, Catholics believe that the Catholic Church has carried on these teachings and I gave the Eucharist and mass as examples. This is a matter of faith just as belief in Jesus is a matter of faith.
If people do not believe that the Catholic Church has the fullness that Scripture refers to, then you do not share the same faith. However, you do still are left with the question of what Church does have the fullness that Scripture makes clear exists:
remain faithful to what you have learned and believed,
hold fast to the traditions, just as I handed them on to you.
stand firm and hold fast to the traditions that you were taught
So Matt and Ahimsaman, I ask you, what Church has carried on these teachings, traditions, and beliefs that St. Paul refers to? What Church is this?
Greg