Because the teachings of the Catholic Church is "another gospel" and not what the apostles taught. Therefore those that teach it are to be considered accursed.
Galatians 1:8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
If Catholics would just show where their beliefs were taught by the apostles there would be no opposition.
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Did God say He would be with His Church until the end of time?
Did God say He would send the Holy Spirit to guide His Church?
Do you believe God or not?
(By the way, where did the apostles teach that it was okay for you to post on the internet?)