Would God leave us without an understanding of Scripture? Does it make sense that God would leave us all on our own, left to figure it out for ourselves? Even with the Holy Spirit always teaching us (John 14:26), people are led to all sorts of different interpretations. It's only sensible that God would give us some tools to help us out.
One tool lies in the beautiful history of our Church. Does salvation come from faith, works, or both? Are Father, Son, and Holy Spirit one in the same? What books & letters should be considered "inspired"? These & many more questions have already been answered by Church councils throughout the ages, led by people far better educated in Scripture & history than any of us. The question of whether or not circumcision is required for salvation is answered by the First Council of Jerusalem in Acts 15!
It also makes sense that God would give us a living voice. He gave the ancient Hebrews the Prophets, He gave the New Testament Israelites Jesus...there's no reason why he would leave us without a living voice, somebody who teaches the Church with the authority of the Holy Spirit! And who would he give this teaching authority to? Whoever "leads the flock", so to speak. The person who leads the Church as a servant of Jesus, the successor of the first person Jesus left in charge (St. Peter), is the logical choice for a living voice to guide the people; hence the long-established doctrine of papal infallibility. (More on that topic; if you want to rebuke me on this point, please read through this first:
Papal Infallibility)
As for discerning on our own: It's helpful to point out that every way Jesus quoted Scripture was with trust in God; as for Satan, every way he quoted Scripture was with an intent to destroy. Jesus told us what the greatest commandment is (Matthew 22:26-30); if our interpretation of the Bible helps us serve this commandment, you should be alright.
I pray this all helps anyone who's interested. Even though there's a lot here that I'm sure will be met with argument, please don't just focus your post on tearing about the Catholic; build up whatever you think is a better alternative. The OP has an excellent point; I'm not trying to divide or prove anyone wrong, I'm just sharing what I believe to be the answer.
May God bless us all & our attempts to follow Him more closely!