We know they don't. If they did believe the full penalty for sins was paid at Calvary and that once you believe The Gospel you are saved for eternity their house of cards collapses. Once they believe these truths they won't assemble out of obligation, but will gather together in fellowship. Once they believe these truths their clergy will no longer be elevated above believers and will no longer be presumed to have supernatural powers.
No matter how many verses of Scripture you quote they will not believe it. Their faith is in their church not The Gospel.
Full penalty (I’ll spare you the dramatic all-caps) payment means the 3rd Commandment turns into a Suggestion. Interesting!
We do not just gather, at Mass, for fellowship, but primarily for Worship of God.
The priest is acting in-persona Christi, In the Person of Christ. It is Jesus Christ who is acting through the priest on our behalf.
Actually the Gospel cam to you though the Catholic church. It was the Catholic Church who protected the scriptures and decided which books were inspired. This through the Teaching Authority given to the Apostles and their decedents, to protect and teach the Truth.
We do believe in the finished work of Jesus Christ, not just for ourselves but for the whole world, including all who post on this site. We believe that Jesus loved us so much, that He not only wanted to die to offer his suffering and death for our salvation, He also wanted to give himself to us, His Church, as a husband gives himself to his wife, so that we might be brought into Him. The Mass is an obligation, because it says in scripture, do not be absent from the assembly as some do, but also because if you knew the reality of Who is present at the Mass, and that Calvary is made available, for us to enter into, with Jesus, it would be unthinkable to miss. We do not just gather for fellowship, but primarily for Worship. I really do not think the issue is the finished work of Christ, because even you believe that you have to repent and ask Jesus to save you(application of the finished work). I think the reason you have a problem, is that you believe once saved, always saved. That is the issue, in my opinion.