The big problem is... they are not a true church. They do not hold to the one true gospel of salvation by grace alone, through faith alone, in Christ alone. They have replaced it with a sacerdotal system via the sacraments, falsely thinking Christ's righteousness can be obtained in this manner. Therefore, over a billion people are deceived and could perish unless they realize the only hope they have is in Christ ALONE.
In this vein, Rob Zins has written an excellent article contrasting the Biblical way of salvation with the Romanist way of salvation. Here's a taste of the article below and a link to the article here.
Taken from "The Heart of the Roman Catholic Religion"
"Christians, trusting in the Bible alone for salvation and sanctification, understand that heaven is given to lost sinners on the basis of faith alone in the finished work of Jesus Christ alone. Such faith consists in confidence that Christ alone, at His cross, suffered and died for all of the sins of His Church which is the Body of Christ. Such a faith completely trusts in the promises of God in Christ Jesus. One such assurance is eternal forgiveness of all sins and punishments based entirely upon the satisfaction of Christ’s death. Such a faith takes the righteousness of Christ as the complete ground of justification. Such a faith grasps Christ’s righteousness immediately. Christians believe there is only One mediator between God and man and that is Jesus Christ. Christians also believe there is no mediator between them and Jesus Christ. The Roman Catholic religion believes itself to be the mediator between man and Jesus Christ. But Christians cannot conceive of a “go-between” and deny the necessity of a “a middle man” between poor lost sinners and Jesus Himself!
This is what separates Rome from the Gospel of Christ. Rome insists upon a priestly led sacramental salvation. In so doing, Rome is outside of the Bible and remains lost in the labyrinth of her own traditions, religious rituals and humanly conceived auto soterism i.e., self salvation. This is not the Christian Gospel. It is not Christianity."