I am a former Catholic and am very familiar with Catholic doctrine.
There is a HUGE difference between Biblical Christianity and CAtholic teaching. Perhaps the biggest difference can be summed up with this term: "The Sufficiency of Christ". Please allow me to explain. Catholicism teaches that Christ did not or could not completely redeem mankind at Calvary, hence the necessity of continuing the work of redemption through the Mass and Eucharist. Biblical Christianity, on the other hand, asserts that Christ completed His perfect work of redemption when He died on the cross, leaving NOTHING for sinful man to do (re: his salvation) except by faith trust that Christ did what He said He did. What?!! You mean to tell me just a simple act of faith can approopriate Christ's death and sin payment on my behalfl? Yes, it's simple yet profound. You see, it gives ALL the glory to Christ and none to sunful me, since I am unable to do anything that would be acceptable to Holy God for my own atonement. ANd remember, God's word says we cannot respond in faith unless the Spirit of God calls us. The good news is that He calls everyone--even the native in the bush who has never heard the name of Christ--read Romans Chapter 1. What will your answer be when Almighty God asks you why He should let you into His heaven? Will it be "I trusted Christ alone to save me" or "Well, God, I've done this, this and this, and oh yes, I've trired to follow the commandments and I attended church faithfully and received the Eucharist too, etc., etc."
The world tells us that you don't get something for nothing and sadly, most world religions (including Catholicism) are in agreement. The good news , i.e., the gospel message, is this: We are all sinners bound for hell but Christ died in our place, making complete atonement for our sins so that God will find us fully acceptable--based on what His Son did NOT on anything we could ever do. That's more than good--that's GREAT AWESOME news!
When you deny the redemption was insufficieint it opens a huge Pandora's box--examples: other mediators, merited salvation, purgatory, tradition and magesterial authority taking precedence over Scripture, indulgences, good works as means to appease God, etc.
One more note on differences: Catholic grace is merited while biblical grace is unmerited. Same word but opposite meaning.
Hate the apostacy---Love the apostate!