They base that view on several things primarily although they vary from sect to sect. Most agree on the following as proving that Catholicism not only contradicts the Bible but is actually anti=-Christian
First and foremost the Pope. They will point out there is no scriptural reference to the Pope whatsoever and therefore, we are in error from square one.
Second the Mass itself. They will point out that when Jesus broke the bread at the last Supper he was merely speaking fuguratively, not literally as the eating of flesh and the drinking of blood would have been repugnant to Jews. It naturally follows that when the Priest consecrates the Host he is obviously in error. Further, they say that by re-presenting the sacrifice at Calvary Catholics completely deny the efficiency and completeness of the sacrifice. If it was sufficient in the first place why must the Priest re-present it?
Third Marian devotion They will point out that Mary was a minor figure, pretty brusquely and rudely treated by Jesus and in no way shape or form the Mother of God as she was a mortal woman.
**Fourth Priesthood itself ** they will point out that there is no evidence that Jesus passed any authority on to the Apostles but if he did somehow, there was no way they could pass it on to anyone else. So, the Sacrament of Reconciliation is worthless as no man can hear and forgive sins, only God.
**Fifth Baptism ** they will point out that there is absolutely no scriptural justification for even thinking that there is such a thing as Original Sin and therefore no reason to believe that Baptism could remove it. Baptism is just an initiation or welcome into the community in their eyes.
Sixth The Sacraments as a whole. They will point out there is absolutely no scriptural basis for the sacraments and therefore they are man made, Some Churches do admit that Baptism and the Eucharist can be considered sacramental in nature.
Seventh Purgatory They will point out there is no scriptural references to such a place or state, remember they don’t accept the Books of the Maccabees as being scriptural, and it firther denies the eficiency of the Sacrifice at Calvary. If we are saved, why must we pay a temporal debt for sins already forgiven?
Basically that is about it. There are others, the Saints etc, but the ones above are the biggies.